simplified middle school chemistry for std viii

New Simplified Chemistry Class 6 ICSE Solutions – Air and Atmosphere

ICSE SolutionsSelina ICSE SolutionsML Aggarwal Solutions

Simplified ChemistryChemistryPhysicsBiologyMathsGeographyHistory & Civics

Exercise

Question 1.
State what do you understand by the term ‘air’. Explain its importance to mankind.
Answer:
Air is a mixture whose composition varies at different intervals of time and different places of the world.
Man cannot survive without air and water and hence air is very essential for survival.

Question 2.
Give a brief account of the discovery of air and the scientists involved.
Answer:

1. John Mayow (1674) proved air has two components – active and inactive components.
2. Lavoisier (1789) named active component as oxygen and inactive component as nitrogen
3. Other scientists (1800’s) discovered noble gases, CO₂, water vapours.

Question 3.
What is meant by the term ‘atmosphere’. State the role played by the atmosphere for the survival of mankind.
Answer:
A blanket of air around earth is called atmosphere. Without atmosphere life would not be possible as atmosphere protects us from harmful gases. We could not live without air present in atmosphere. In absence of the atmopshere, the earth would get so cold at night that we would not be able to survive. No CO₂ and N₂ for plants without atmosphere.

Question 4.
Explain in brief the different layers of the atmosphere and the basic functions of the main layers.
Answer:

Question 5.
Describe a simple experiment with the help of a diagram to show that :

(a) Air occupies space
(b) Air has mass
(c) Air exerts pressure
(d) Air is highly compressible

Answer:
(a) Air occupies space :

• Press an inverted glass
• Tilt the tumbler to one side, tumbler over the water.

Observation : On tilting the tumbler, bubbles of air are seen coming out.
Water slowly enters the tumbler to take place of the trapped air in the tumbler.
Conclusion : Air occupies space (the space in the tumbler was occupied by air).

(b) Air has mass :

• An inflated balloon is placed on one side of the scale.

Observation : It is seen that the scale moves towards one side.
It moves on the side on which the inflated balloon was placed.
Conclusion : Air has mass.

(c) Air exerts pressure :

• Take an open can, fill it with water and heat it. On formation of steam, place the cap on tightly and keep the can aside.

Observation : On condensation of the steam the pressure inside the can reduces.
The air outside, exerts pressure on the can which crumples slowly.
Conclusion : Air exerts pressure.

(d) Air is highly compressible :

• A leak proof syringe filled completely with air only (i.e. empty) is taken.

Observation : On pressing the piston of the syringe, the piston moves inwards.
Air (gas) inside the syringe has maximum inter-molecular space and is highly compressible.
Conclusion : Air is highly compressible.

Question 6.
Name the components of air with their approximate percentage by volume in air. Does the percentage by volume of each component remain the same, in the atmosphere of different parts of the world. Explain with reasons.
Answer:
Main components of air are :

1. Nitrogen
2. Oxygen

Composition of air by volume :

1. Nitrogen – 78% or 4/5 th
2. Oxygen 21% or 1/5 th
3. Carbon dioxide 0.03 – 0.04%
4. Inert gases 0.9%

Air is a mixture whose composition varies at different intervals of time and different places of the world.

Question 7.
With the help of a labeled diagram, describe a simple experiment to show the presence of oxygen and nitrogen in air using a piece of white phosphorus.
Answer:
Activity to show the presence of oxygen and nitrogen in air using a piece of white phosphorous.

1. A trough is taken and fdled with water.
2. Over it is placed a bell-jar marked with five equal parts.
   
3. A crucible containing white phosphorus is placed on a cork.
4. The cork is made to float on the water.
   
5. A heated iron rod is them taken.
6. The phosphorus is ignited with the heated rod.

Observation : Dense white fumes of phosphorus pentoxide (P₂O₅) are formed when the phosphorus (P) burns in the active component of air (oxygen).

The rise in the level of water in the bell-jar is by – 1/5


The active component of air i.e. oxygen (1/5 of air) is used up in burning.
The remaining inactive component of air i.e. nitrogen (4/ 5 of air) is not used up in burning.

Question 8.
You are given a test tube with two outlets and a bottle of lime water. Using the same, how would you demonstrate experimentally the presence of carbon dioxide in air.
Answer:
Procedure :

1. Take lime water in a test tube with two outlets ‘A’ and ‘B’ as shown above.
2. Air is sucked slowly through the outlet ‘B’.
3. Air moves in through end ‘A’ and passes through the lime water.

Observation : The line water in the test tube turns milky.
Conclusion : Air which is sucked in from and ‘A’ contains carbon dioxide which turns the lime water milky. Hence, air contains – carbon dioxide.

Question 9.
Give a reason why water droplets appear on the outer surface of a tumbler containing ice.
Answer:
Water vapour present in air, condenses on the cooler surface of the tumbler. Hence they condenses into tiny water droplets.

Question 10.
Explain the importance of nitrogen of the air for plant growth.
Answer:
Nitrogen of air is converted to soluble nitrogenous compounds in the soil in the presence of air and moisture. These are absorbed by plants and converted to plant proteins.

Question 11.
Give a reason why nitrogen is filled in food packets and not oxygen.
Answer:
Nitrogen being inert or unreactive is filled into food packets to drive out the oxygen and reduce bacterial growth. The food package then stays well preserved.

Question 12.
State what would happen, if the air above the earth contained mainly oxygen and no nitrogen.
Answer:
Nitrogen is utilised by plants for their growth and development.
Free nitrogen of the air is converted to nitrogen compounds which are absorbed by the plants and converted to plant proteins. In the absence of nitrogen no plants will be survived on the earth.

Question 13.
State the utility of oxygen for respiration in

(a) living organisms
(b) plants.

Answer:
(a) Respiration in living organisms :
Respiration— It is a process whereby living things

1. Use oxygen from their air to oxidize food substances mainly glucose, in their body cells.
2. Release energy in the form of heat.
3. Carbon dioxide and water vapour are also produced and released in the exhaled air.
   
   

Process—

1. Inhaled air — Oxygen from inhaled air dissolves in the blood stream in the lungs. It is carried to the cells where the food materials i.e. glucose is oxidized by the oxygen.
2. Exhaled air — Carbon dioxide, water vapour and heat energy diffuse out of the lungs as exhaled air.
3. Energy liberated — The energy liberated is utilized for metabolic activities and maintains the body temperature.

(b) Respiration in plants :

1. Plants respire during the day and night.
2. Plants— Respire through tiny pores on the surface of the leaves called ‘stomata’.
3. The stomata help in taking in oxygen of the air and giving out carbon dioxide. Thus, they function as respiratory organs.
4. Plants do not perform bodily activities and hence need less energy than animals. The rate of respiration in plants is thereby slower than in animals.

Photosynthesis in plants :
Photosynthesis is a process by which green plants prepare their own food in the presence of sunlight and chlorophyll.

1. In the leaf of a green plant are present leaf cells which contain chloroplasts in which is present a green pigment called chlorophyll.
2. During photosynthesis which occurs during daytime, carbon dioxide and water in the presence sunlight and chlorophyll is changed into glucose and oxygen is released out.
   
3. Hence oxygen in the air is renewed by photosynthesis and therefore the amount of oxygen in air does not get depleted or removed from air.

Question 14.
Compare respiration and combustion-both involving oxygen of the air.
Answer:
Respiration is a process whereby living organisms

1. Use oxygen from the air to oxidize food substances mainly glucose, in their body cells.
2. Release energy in the form of heat.
3. Carbon dioxide and water vapour are also produced and released in the exhaled air.
   

Combustion or burning, involves oxidation i.e. combination of substances like fuels with oxygen or air generally resulting in production of heat and light.

Carbon dioxide is released into the air as a result of all burning.

Question 15.
Explain the importance of carbon dioxide for
(a) photosynthesis (b) warming the earth’s environment. State what would happen if excess carbon dioxide as a pollutant is released into the atmosphere.
Answer:
(a) During photosynthesis which occurs during daytime, carbon dioxide and water in the presence of sunlight and chlorophyll is changed into carbohydrates and oxygen is released out.

Hence oxygen in the air is renewed by photosynthesis and therefore the amount of oxygen in air does not get depleted or removed from air.
(b) Infrared (I.R.), visible rays and ultra violet (U.V.) rays are present in the earth’s surface.
The reflected U.V. rays from the earth’s surface pass through the carbon dioxide in the atmosphere, but the
I. R. rays are prevented by the carbon dioxide from being radiated out of the earth’s surface. This results in the earth’s environment staying warm.
The thick CO₂ layer prevents the heat radiations from being radiated out which reason :
Reason : It results into rise in global temperature.

Question 16.
State in brief how water vapour in the atmosphere determines the climatic conditions.
Answer:
Water vapour in the atmosphere in excess :

(a) Minimizes the rate of evaporation.
(b) Produces rain, mist etc. thereby determining climatic conditions.

The rain serves as a natural source of water for plants and animals.

Question 17.
Give a reason why two different rare (inert) gases find application in advertisement signs and fluorescent bulbs.
Answer:

1. Neon – In neon sign advertisements
   A brilliant red glow is seen on passage of an electric current through neon gas at low pressure.
   The colour can be changed by mixing mercury vapour and argon with neon.
2. Xenon – In fluorescent bulbs flash bulbs and lasers. It emits intense white light in discharge tubes.

Question 18.
A mixture has a variable composition. Give three other reasons why air is considered a mixture and not a compound.
Answer:
Air is a mixture because :

1. Air has no formula, a mixture has no formula whereas compound has a formula.
2. No energy changes are involved to form air from various gases.
3. When air is formed out of its constituents no change in mass nor change in volume takes place.
4. Properties of air vary from place to place and time to time. i.e. there is more CO₂ in towns as compared to villages where more oxygen prevales as compared to towns.
5. Components of air can be separated by simple physical methods.

Question 19.
State a reason why there is a balance in the amount of carbon dioxide in the air, even though carbon dioxide is released into the atmopshere by various processes.
Answer:
By the way of photosynthesis in plants which which occurs during daytime, carbon dioxide and water in the presence of sunlight and chlorophyll is changed into carbohydrates and oxygen is released out.


Hence, oxygen in the air is renewed by photosynthesis and therefore the amount of oxygen in air does hot get depleted or removed from air.

Question 20.
Name three different appliances where air is utilized.
Answer:

1. Vehicles – Run on tyres which are inflated with air.
2. Brake mechanisms – Of trains and other machines, work on compressed air.
3. Pumps and siphons – Work on air pressure.

Objective Type Questions

Q.1. Match the statements in List I with the correct answers in List II.


Answer:

Q.2. The diagrams below represents an experiment to show the presence of a – component of air.

Question 1.
State why air is sucked from – outlet ‘B’ and not from outlet ‘A’.
Answer:
Because if we suck from ‘A’, lime water will be sucked out.

Question 2.
State why the air is sucked ‘slowly’ from the outlet ‘B’
Answer:
Because if air is sucked fast, lime water will also be sucked out.

Question 3.
State the observation seen after completion of the experiment.
Answer:
Lime water turns milky.

Question 4.
State the reason for the above observation seen.
Answer:
The reason is the formation of CO₂.

Question 5.
Name another component which if present in air as a pollutant – would have given a similar observation.
Answer:
Sulphur dioxide.

Q.3. Complete the statements given below by filling in the blanks with the correct word/s from the word/s in bracket.

Question 1.
The ____ [atmosphere /troposphere/stratosphere] is a layer which extends upto about 10-50 kms. above the earth.
Answer:
The stratosphere is a layer which extends upto about 10-50 kms. above the earth.

Question 2.
____ [helium/hydrogen sulphide] is an example of a pollutant present in air.
Answer:
Hydrogen sulphide is an example of a pollutant present in air.

Question 3.
The component of air used in photosynthesis is ____ and the products of photosynthesis is ____ [oxygen/carbon dioxide/nitrogen].
Answer:
The component of air used in photosynthesis is carbon dioxide and the products of photosynthesis is oxygen.

Question 4.
The product formed which is common to both combustion and respiration is ____ [oxygen/ carbon dioxide/nitrogen dioxide]
Answer:
The product formed which is common to both combustion and respiration is nitrogen dioxide.

Question 5.
The component of air which is variable in air above sea level and in air in general ____ [oxygen/ nitrogen/water vapour].
Answer:
The component of air which is variable in air above sea level and in air in general water vapour.

Q.4. Name the following :

Question 1.
The product formed when phosphorus burns in oxygen.
Answer:
Phosphorus pentoxide (P₂O₅).

Question 2.
The layer of atmosphere which contains the ozone layer.
Answer:
Stratosphere.

Question 3.
The active component of air which supports combustion and is used up in burning.
Answer:
Oxygen.

Question 4.
The main, rare (inert) gas present in air.
Answer:
Argon.

Question 5.
The component of air which is present more in industrial areas.
Answer:
Carbon monoxide, sulphur dioxide, oxides of nitrogen, hydrogen sulphide, dust particles.

Question 6.
The type of bacteria which directly absorb nitrogen from the air and converted soluble nitrates in the soil.
Answer:
Symbiotic bacteria.

Question 7.
The product of respiration released in exhaled ajr, other than carbon dioxide.
Answer:
Water vapour and energy.

Question 8.
An inert gas which has a low boiling point and is used for producing very low temperatures.
Answer:
Helium.

Question 9.
A process which removes carbon dioxide from the air.
Answer:
Photosynthesis.

Question 10.
The component of air which does not support combustion.
Answer:
Nitrogen.

Q.5. State whether the following statements are ‘true’ or ‘false’. If false write the correct statement.

1. The mesosphere in the atmosphere, contains most of the air, fit for respiration.
Ans. False.
Correct : The troposphere in the atmosphere, contains most of the air, fit for respiration.

2. The percentage of carbon dioxide in the air is between 0.2 to 0.4%.
Ans. True.

3. Carbon dioxide, water vapour and energy in the form of heat are evolved during respiration.
Ans. True.

4. Photosynthesis is a process by which green plants prepare their own food in the presence of sunlight and chlorophyll.
Ans. True.

5. Air is a mixture and not a compound since the components of air cannot be separated by physical methods.
Ans. False :
Correct : Air is a mixture and not a compound since the components of air – can be separated by physical methods.

Q.6. Give word equations for the following conversions.

Question 1.
Nitrogen of the air to nitric oxide.
Answer:

The nitric oxide reacts with oxygen to give nitrogen dioxide.

Question 2.
Nitric oxide to nitrogen dioxide.
Answer:
Nitric oxide + Oxygen → Nitrogen dioxide
The nitrogen dioxide further reacts with oxygen and water vapour of the air to give nitric acid.

Question 3.
Nitrogen dioxide to nitric acid.
Answer:
Nitrogen dioxide + Oxygen + Water → Nitric acid
The nitric acid formed comes down with the rain and combines with the carbonates in the soil to form soluble nitrates.

Question 4.
Nitric acid to soluble nitrates in the soil.
Answer:
Calcium carbonate + Nitric acid → Calcium nitrate + water + carbon dioxide
The water soluble calcium nitrates are absorbed by the plants and converted to plant proteins.

Question 5.
Carbon dioxide in air to carbohydrates in plants.
Answer:

Carbon dioxide and water in the presence of sunlight and chlorophyll is changed into carbohydrated and oxygen is released out.

New Simplified Chemistry Class 6 ICSE Solutions – Water

ICSE SolutionsSelina ICSE SolutionsML Aggarwal Solutions

Simplified ChemistryChemistryPhysicsBiologyMathsGeographyHistory & Civics

Exercise

Question 1.
State the sources of water

(a) on the earth’s surface
(b) below the earth’s surface.

Answer:
(a) Sources of water on the earth’s surface are —

1. Snow, frost — Snow and frost are the natural forms of water present. They are found in the solid state.
2. Rain water — The purest form of natural water almost free from impurities is rain water.
   Rain water may dissolve oxygen, nitrogen & carbon dioxide gas forming weak acids. In industrial regions, nitrogen dioxide and sulphur dioxide evolved may dissolve in rain water forming nitric acid & sulphuric acid which causes acid rain.
3. River water — It is one of the impure forms of natural water since most forms of surface water enters into river water. It contains impurities such as sand partides, organic matter, bacteria, mineral salts which dissolve after soil erosion & dissolved gases.
4. Lake water — It is another impure form of natural water which also contains impurities and other soluble salts.
5. Sea water — The most impure form of natural water containing over 3% soluble salts including sodium chloride. It also contains salts of calcium & magnesium.

(b) Sources of Water below the earth’s surface are —

1. Well water — Above the impervious rocky layers, of the earth’s surface is well water which contains soluble impurities.
2. Spring water — Natural water accumulated above the rocky layers of the earth which forcefully comes out under pressure from the earth’s surface is spring water & contains soluble salts & minerals.

Question 2.
Give the importance of water in

(a) life processes
(b) household purpose
(c) fire fighting
(d) transportation.

Answer:
The importance of the Water is as follows :

(a) Life processes — Water is used by all plants, animals & humans for carrying out various metabolic processes including photosynthesis by plants & excretion by animals & humans.
(b) Household purposes — Water finds numerous applications, such as watering plants, washing clothes, cooking, bathing, cleaning etc.
(c) Fire fighting — Water is used for extinguishing fires either directly or as a constituent in a fire extinguisher.
(d) Transportation — Water serves as a habitat for marine life i.e. preferred place for an organism to live.

Question 3.
Explain how water plays an important role in

(a) industry
(b) agriculture.

Answer:
(a) Uses of Water in Industry :

1. Water generates electricity in hydroelectric power stations.
2. Water generates steam in boilers, used for various industrial purposes.
3. Water finds application in chemical & other industries for cooling & cleaning operations.

(b) Uses of Water in Agriculture :

1. In agriculture water finds importance in irrigation, production of crops & as a medium for spraying pesticides.

Question 4.
Give the occurrence of water in the three different states i.e. solid, liquid and gaseous.
Answer:
The Occurrence of water in three different states are :

1. Solid state — As snow and frost.
2. Liquid state — In sea water, river water and lake water.
3. Gaseous state — As water vapour in air the amount depending on climatic conditions.

Question 5.
Draw a labelled diagram to show the change of state of water from solid state to liquid state to gaseous state starting from ice.
Answer:

Question 6.
Explain the term water cycle. State the main points to show how water moves from the earth’s surface to the atmosphere and back to the earth’s surface as rain.
Answer:
The water cycle is a natural process by which the circulation of water takes place from the earth’s surface to the atmosphere and back to the earth’s surface as rain water.
The Process of water cycle is discussed below :
From The Earth’S Surface – To The Atmosphere

1. Evaporation — The sun’s rays fall on the earth & warm its surface & the air above it.
   The heat evaporates the water from the streams, rivers & the sea.
2. Water [mainly in the form of water vapour] is also added to the atmosphere by —
   (a) Respiration by living organisims Glucose + Oxygen → carbon dioxide + water + energy
   (b) Burning of fossil fuels —
   
3. Formation of clouds — The water vapour along with the warm air rises upwards, where at higher altitudes it condenses into small droplets of water forming clouds.

Back To The Earth’S Surface – As Rain Water

1. Formation of rain — The clouds float in the atmosphere & when the size of the water droplets increases they fall down on the earth as rain water.
2. Rain water falls into streams — The rain water is absorbed by the soil collects underground & flows into streams.
3. Stream water enters rivers & seas — The stream water finds its outlets into rivers & later enters into the seas & oceans.
4. River & sea water evaporates forming clouds and thus continuing the water cycle.

Question 7.
Give a reason why water is considered a universal solvent.
Answer:
Water is a polar covalent compound. When it comes in contact with any substance it breaks the electrostatic forces holding the molecules of that substance. Thus, the molecules break loose from the substance and hence dissolve in water. Thus, water is called a universal solvent and an alkali is not.

Question 8.
Define the term –

(a) solute
(b) solvent
(c) solution with reference to addition of sodium chloride to water.

Answer:

(a) Solute — The substance which dissolves or disappears in the solvent i.e. liquid to form a solution is called a solute. e.g. sodium chloride.
(b) Solvent — The liquid or medium of dissolution which allows the solute to dissolve in it, so as to form a solution is called a solvent, e.g. water.
(c) Solution — A homogenous mixture of a solute in a solvent is called a solution.

Question 9.
Draw a neat labelled diagram of addition of copper sulphate to water. Label the solute, solvent and solution in the same.
Answer:

Question 10.
From the following substances given below state which will form a solution in the same.

(a) sodium carbonate
(b) calcium carbonate
(c) charcoal powder
(d) sodium sulphate
(e) table salt
(f) powdered particles of lead from a lead pencil
(g) iron powder
(h) copper filings
(i) sand particles
(j) honey

Answer:

(a) sodium carbonate
(d) sodium sulphate
(e) table salt
(j) honey

Question 11.
Name two gases each

(a) which are soluble in water
(b) which are insoluble in water.

Answer:

(a) The gases which are soluble in water are Carbon dioxide and chlorine
(b) The gases which are not soluble in water are Nitrogen, hydrogen

Question 12.
If ‘X’ g. of potassium nitrate is added to 100 g. of water at 60°C and the salt dissolves completely then —

(a) is ‘X’ g. the solubility of potassium nitrate at 60°C.
(b) is the solution formed – saturated or unsaturated
(c) if on addition ‘X’ + ‘Y’ g. of potassium nitrate to the same amount of water at the same temperature and the solute now just remains behind after stirring then –
(d) is the solution now – saturated or unsaturated
(e) is ‘X’ + ‘Y’ g. the solubility of potassium nitrate.

Answer:
Add ‘X’ g. of solute i.e. potassium nitrate to 100 g. of water 60°C.

1. Stir the solute i.e. potassium nitrate in water thoroughly.
2. ‘X’ g. of the solute completely dissolves in water.
3. Add more solute and again stir thoroughly.
4. The solute continues to dissolves.
5. Water i.e. the solvent can dissolve more of the solute at the given temperature.
6. The solution is therefore is said to be unsaturated.

Add more solute to water till on adding an amount ‘X + Y’ g. of the solute i.e. potassium nitrate to 100 g. of water at 60°C.

1. The solute just remains behind after stirring.
2. The solution is now saturated.

A saturated solution cannot dissolve more of the solute at a given temperature.

Question 13.
State whether the following statements are true or false. If false write the correct statement.

(a) Solubility of most solids – decrease with increase in temperature.
Answer:
False.
Correct — Solubility of most solids – increase in temperature.

(b) Distilled water is potable water.
Answer:
False.
Correct — Drinking water is potable water.

(c) The process to remove germs in water is also called sterilization.
Answer:
True.

Question 14.
Differentiate between chemical pollution and thermal pollution.
Answer:
(a) Chemical pollution — A large number of industrial chemicals which include chemicals from paint, textile & dyestuff industry & various acids & salt solutions enter into water when discharged as industrial wastes.

• Chemical pollutants include — Metallic salt solutions of mercury & lead which cause heavy devastation of marine & plant life.
• Agricultural wastes include — Poisonous pesticides namely fungicides & insecticides which may also enter underground water through the soil.

(b) Thermal pollution — Certain industries such as the iron & steel industry & numerous chemical plants use large amounts of water for varied functions.

• The discharged waste water after going through technical processes – is rendered hot & on entering streams of natural water – enhance growth of harmful biological organisms.

Question 15.
State some important steps to avoid pollution of water.
Answer:
Steps to avoid pollution of water are :

(a) Harmful wastes such as oils & chemicals should not enter into the water.
(b) Proper toilets & sewage systems should be used to prevent human excreta, containing disease causing organisms to enter into the water.
(c) Washing of clothes & bathing should be avoided near water sources.
(d) Planting of trees near water sources including river banks also minimizes pollution.
(e) To minimize thermal pollution the water should be cooled before being discharged as a waste.
(f) Man should be made aware through various awareness programmes & media about the harmful effects of water pollution & ways to control it.

Question 16.
State what is meant by the term ‘conservation of water’. State a few water saving methods, which may be used in the home to conserve water.
Answer:
Conservation of water is the means of preventing wastage of water so that clean water can be obtained by preventing pollution of water and by protecting the sources of water.
Need for conservation — Inspite of large quantities of water on the earth’s surface only a small percentage is potable water fit for human consumption and household purposes.
The need for water is ever increasing and hence all sources of water need to be conserved.
The various methods to conserve the water are :

1. Well should be covered and washing and cleaning should be prevented near a well.
2. Water saving devices must be used in homes

(a) such as closing running taps and using smaller cisterns in toilets.
(b) checking all leakages in household pipes.
(c) turning off the water tap while brushing teeth and while washing hands.
(d) using less electricity, since power plants also consume substantial amount of water.

Question 17.
Give a reason why :

(a) Conservation of water is essential in spite of the fact that three fourth of the earth’s surface is covered by water.
(b) Polluted water causes disease.
(c) Drip irrigation helps in conservation of water.

Answer:

(a) Because only a small percentage is potable water fit for human consumption and household purposes.
(b) Polluted water acts as a carrier for germs which causes various diseases.
(c) Drip irrigation in agriculture utilizes supply of water in small quantities.

Objective Type Questions

Q.1. Complete the statements by filling in the blanks with the correct word/s.

Question 1.
____is the purest form of water. It may dissolve gases like ____ forming weak acids, (sea water/ lake water/rain water ; chlorine/ammonia/carbon dioxide)
Answer:
Rain water is the purest form of water. It may dissolve gases like carbon dioxide forming weak acids.

Question 2.
Water generates ____ in hydroelectric power stations and ____ in boilers for industrial applications, (carbon dioxide/steam/heat/electricity)
Answer:
Water generates electricity in hydroelectric power stations and steam in boilers for industrial applications.

Question 3.
Water is added to the atmosphere by ____ and ____ (burning of fossil fuels/condensation of vapour/ respiration by living organisms/photosynthesis)
Answer:
Water is added to the atmosphere by burning of fossil fuels and respiration by living organisms.

Question 4.
If potassium nitrate is added to water in a beaker to give a homogeneous mixture, then potassium nitrate is referred to as the ____ water as the ____ and the homogeneous mixture as the ____ (solution/ solute/solvent/saturated solution).
Answer:
If potassium nitrate is added to water in a beaker to give a homogeneous mixture, then potassium nitrate is referred to as the solute, water as the solvent and the homogeneous mixture as the solution.

Question 5.
Water fit for human consumption and drinking purposes is called ____ (distilled water/potable water/rain harvested water).
Answer:
Water fit for human consumption and drinking purposes is called potable water. 

Q.2. State whether the following statements are true or false. If false, write the correct statement.

1. Sea water contains salts of calcium and magnesium.
Ans. True.

2. Water finds application as a means of transporting goods.
Ans. True.

3. On boiling water exists in the liquid state.
Ans. False.
Correct — On boiling water exists in the gaseous state.

4. Respiration uses up water from the atmosphere.
Ans. False.
Correct — Respiration add up water to the atmosphere.

5. Well water exists below the impervious rocky layers of earth.
Ans. False.
Correct — Well water exists above the impervious rocky layers of earth.

6. Sodium carbonate and sodium sulphate do not form an aqueous solution.
Ans. False.
Correct — Sodium carbonate and sodium sulphate form an aqueous solution.

7. If ‘X’ g. of solute is added to 100 g. of water at t°C and the solution formed is a saturated solution, then ‘X’ g. is the solubility of the solute.
Ans. True.

8. Purification of water is carried out to remove – dissolved gases e.g. carbon dioxide and dissolved minerals like magnesium salts.
Ans. True.

9. Water from rivers and lakes is – potable water.
Ans. False.
Correct — Water from rivers and lakes is impure water.

10. Chemical pollutants include metallic salt solution of mercury and lead.
Ans. True.

Q.3. Match the statements in List 1,1-10 with their correct answer in List II, A to J.


Answer:

Q.4. Explain the meaning of the following terms :

1. Rain water harvesting
2. Drip irrigation in agriculture
3. Saturated solution
4. Conservation of water
5. Gaseous state of water in air

Answer:

1. Rain water harvesting which is a means of utilizing rain water instead of allowing it to be wasted to be conducted by building tanks or pits in low lying areas and collecting roof top rain water through pipes into tanks.
2. Drip irrigation in agriculture utilizes supply of water in small quantities.
3. A saturated solution cannot dissolve more of the solute at a given temperature.
4. Conservation of water is the means of preventing wastage of water so that clean water can be obtained by preventing pollution of water and by protecting the sources of water.
   Polluted water acts as a carrier for germs which causes various diseases.
5. Gaseous state of water in air : Gaseous State as water vapour in air the amount depending on climatic conditions.

Q.5. Name the following :

Question 1.
A chemical used during chlorination of water.
Answer:
Chlorine.

Question 2.
An agricultural pollutant in water.
Answer:
Insecticides.

Question 3.
A solid, ‘natural form of water’.
Answer:
Snow.

Question 4.
The natural process by which circulation of water takes place from earth’s surface to atmosphere and back to earth’s surface.
Answer:
Water cycle.

Question 5.
The liquid or medium of dissolution which allows the solute to dissolve in it.
Answer:
Water (solvent).

New Simplified Chemistry Class 6 ICSE Solutions – Matter

ICSE SolutionsSelina ICSE SolutionsML Aggarwal Solutions

Simplified ChemistryChemistryPhysicsBiologyMathsGeographyHistory & Civics

Exercise

Question 1.
Explain the term ‘matter’. One kind of matter can be distinguished from another by its physical properties and chemical properties. State the main physical properties of matter.
Answer:
Matter is the basic substance of which all materials are made of

• Physical Properties — They are those properties which include state, colour, odour, density etc.
• Chemical Properties — They are properties which include reactions of different materials with different chemical.

Physical Properties of Matter are :

1. Colour : All matter an be distinguished by their varied – colours.
2. Odour : Matter shows variation in odour or smell.
3. Solubility : Matter may vary in solubulity in water or other solvents.
4. Melting & Boiling Points : Substances variation in their melting and boiling points.

Question 2.
The three main states of matter are solids, liquids and gases. Compare the three states with reference to the following characteristics of matter –

(a) volume
(b) shape
(c) compressibility
(d) diffusion.

Answer:

Question 3.
Matter in any state is composed of particles. Compare the three states of matter i.e. solids, liquids and gases with reference to :

(a) intermolecular space
(b) intermolecular force of attraction
(c) movement of particles

Answer:

Question 4.
Describe simple experiments to prove that – solids

(a) occupy space
(b) have mass
(c) have a definite volume

Answer:
(a) occupy space : solid-occupies space –
‘A’-A measuring cylinder is filled with water to a particular mark- ‘A’.
‘B’-A piece of wooden block- is immersed inside the measuring cylinder. The water level rises up.
‘C’- On removal of the block- the water level in the measuring cylinder falls down back to the mark – ‘A’
Conclusion : The block pushes the water out and occupies its space, hence all solids occupy space.

(b) Have mass : A solid has mass
A : A simple scale – is taken, as shown below
B : A solid is placed – on one side of the scale, causes the scale tilts towards one side.
Conclusion : The scale tilts due to the mass of the solid, hence all solids have mass.

(c) Have a definite volume : A solid placed in any container filled with water displace the same amount of water.
Conclusion : Solid displace water and retain their own volume, hence all solids have a definite volume.

Question 5.
Describe simple experiments to prove that – liquids

(a) have mass
(b) have a definite volume
(c) have no definite shape

Answer:
(a) have mass — a liquid occupies mass
Experiment : A liquid placed on one side of the scale, causes the scale to tilt towards one side.
Conclusion : The scale tilts due to the mass of the liquid, hence all liquids have mass.

(b) have a definite volume —
Experiment : A liquid in a measuring cylinder can be poured into any container. The volume of liquid in the container, is the same as that in the measuring cylinder.
Observation : The volume of liquid in the container, is the same as that in the measuring cylinder. Hence, all liquids have a definite volume.

(c) have no definite shape —
Experiment : A liquid poured into any container takes up the shape of each container.
Conclusion : All liquids have no definite shape

Question 6.
Describe simple experiments to prove that – gases

(a) occupy space
(b) have mass
(c) have no definite volume or shape

Answer:
(a) occupy space —
Experiment :

• ‘A’ – A glass beaker or bowl is half filled with water.
• ‘B’ – An empty glass tumbler [which of course contains air] is inverted an lowered inside it.
• ‘C’ – On tilting the tumbler, air is displaced and bubbles are seen coming out.

Conclusion : Air or gases occupy space.

(b) Have mass —
Experiment :

• An inflated balloon placed on one side of the scale causes it to tilt towards one side.

Conclusion : The scale tilts due to the mass of the gas, hence all gases have mass.

(c) Have no definite volume or shape —
Experiment :

• A gas takes up the volume of any enclosed space filling it up completely.

Conclusion : Gases take up any volume and hence all gases have no definite volume.

Question 7.
Explain the term ‘Interconversionof matter’. With reference to ice, water and water vapour show diagrammatically the change of state of matter from solid to liquid to gaseous and back to original state.
Answer:
INTERCONVERSION OF MATTER :
“Change of ,state of matter from one state to another state and back to its original state is called inter – conversion of matter.”
The diagram showing the Change of State of Matter :

(a) Ice To Water (Melting) : Conversion of a solid into a liquid on heating e.g. ice to water.
(b) Water To Water Vapour (Vaporisation) : Conversion of a – liquid into vapours (or gas) on heating e.g. water to water vapour.
(c) Water Vapour To Water (Liquefaction Or Condensation) : Conversion of VAPOUR (or gas) into a LIQUID on cooling e.g. water vapour to water.
(d) Water To Ice (Solidification Or Freezing) : Conversion of a LIQUID into a SOLID e.g. water to ice.

Question 8.
Explain the terms

(a) melting
(b) vaporization
(c) condensation
(d) freezing
(e) melting point
(f) boiling point.

Answer:
Matter can change from solid to liquid to gaseous state and back to solid state. This is called change of state of matter.

(a) Melting — The process of conversion of a solid into liquid on heating.
e.g. Ice to water. k
(b) Vaporization — The process of conversion of a liquid into vapour on heating.
e.g. Water to water vapour.
(c) Condensation — The process of conversion of vapour into a liquid.
e.g. Water vapour to water.
(d) Freezing — The process of conversion of a liquid into a solid.
e.g. Water to ice.
(e) Melting point — The constant temperature at which a solid melts into a liquid.
M.P. of ice – 0°C.
(f) Boiling point — The constant temperature at which a liquid starts boiling.
B.P. of water – 100°C.

Question 9.
State what would you observe if

(a) sugar is added to pebbles take in a plastic beaker
(b) sand is added to glass balls in a beaker. What would you conclude from this imaginative demonstration.

Answer:

• Experiment : Add sugar to pebbles taken in a plastic beaker or sand to glass balls in a beaker.
• Observation : The sugar or the sand goes into the space between the pebbles & the glass balls respectively.
• Conclusion : An imaginative demonstration to show that intermolecular spaces between particles are occupied easily.

Question 10.
With the help of a simple diagram how would you show that – solids expand on heating.
Answer:
Solids expand on heating can be shown by two different experiments. The experiments are :

• Experiment A : An iron bar is taken and its length measured accurately.
  The iron bar is then slowly heated for a certain period of time & measured accurately [with a vernier calipers or otherwise]
• Observation : It is observed that the iron bar has Increased in length on heating.
• Experiment B : A simple ball & ring apparatus is taken, which consists of a metal ball which can just pass through the circular metal ring.
  The metal ball is then heated for a certain period of time.
• Observation : After heating, the metal ball does not pass through the ring, since on heating it has expanded in size and hence cannot pass through the ring.
• Conclusion : All solids expand on heating.

Question 11.
Give reasons for the following :

(a) Solids have a definite shape and are highly rigid while gases have to definite shape and are least rigid.
(b) Sugar can be distinguished from talcum powder using water.
(c) Water on freezing turns into ice.
(d) A bottle of perfume on opening evolves an odour which can be sensed over a long distance.

Answer:

(a) Solids have very closely packed atoms with minimum spaces between them while gases have atoms which are for apart with maximum spaces between them. Solids have a definite shape and are highly rigid while gases have no definite shape and are least rigid.
(b) Sugar is soluble in water whereas talcum powder is not.
(c) Every pure substance has a fired melting point or boiling point.
(d) A bottle of perfume on opening evolves an odour because gases diffuse very easily and odour spreads over a large distance.

Question 12.
Complete the statements given below by selecting the correct word/s.

(a) Solids and liquids have a definite ____ but gases do not. [mass, shape, volume]
Answer:
Solids and liquids have a definite volume but gases do not.

(b) The space between atoms in is maximum while in ____ is minimum.[solids, liquids, gases]
Answer:
The space between atoms in gases is maximum while in solids is minimum.

(c) Conversation of a vapour into a liquid is called ____ [vaporization, condensation, freezing]
Answer:
Conversation of a vapour into a liquid is called condensation.

(d) ____ is an example of a crystalline substance. [wax, sugar, tea]
Answer:
Wax, sugar is an example of a crystalline substance.

Question 13.
State which of the following statements are false. If false write the correct statement.

(a) Solids are highly compressible and rigid.
Answer:
True.

(b) Atoms/molecules in gases move only about their own positions.
Answer:
False.
Atoms/molecules in gases move every where.

(c) The conversion of water to ice is called freezing.
Answer:
True.

Objective Type Questions

Q.1. Fill in the blanks with the correct word/s from the bracket.

1. From the three states of matter, solids expand the least.
2. Brownian movement is maximum in gases.
3. Cohesive forces are negligible in gases.
4. Matter can change from one state to another by change in temperature or pressure.
5. The space between atoms [molecules] of solids is minimum.
6. Intermingling of molecules is called diffusion.
7. Ice on absorption of heat converts to ‘X’ a process called melting. ‘X’ changes to water vapour on heating. Water vapour changes back to ‘X’ on condensation. The constant temperature at which ice changes into ‘X’ is called its fusion point.

Q.2. State which of the following are physical properties of a substance.

Question 1.
Chlorine gas has a – strong irritating odour.
Answer:
Physical.

Question 2.
Sodium nitrate is soluble in water, but calcium carbonate is not.
Answer:
Physical.

Question 3.
Magnesium reacts with dilute hydrochloric acid, liberating hydrogen gas.
Answer:
Chemical.

Question 4.
Manganese dioxide, a catalyst which alters the rate of a chemical reaction is black in colour.
Answer:
Chemical.

Question 5.
The melting point of ice is 0°C.
Answer:
Physical.

Question 6.
Lead chloride reacts with barium sulphate to give a white precipitate of lead sulphate.
Answer:
Chemical.

Question 7.
Water acidified with dilute sulphuric acid is a good conductor of electricity.
Answer:
Physical.

Question 8.
Naphthalene on heating directly turns into vapour.
Answer:
Chemistry.

Question 9.
Hydrogen sulphide gas has a strong rotten egg odour.
Answer:
Physical.

Question 10.
Sulphur is a yellow amorphous powder insoluble in water.
Answer:
Physical.

Q.3. Match the characteristics of the three states of matter in List I with their correct answer from List II.

Answer:

Q.4. Match the arrangement of atoms in the three states of matter in List I with the correct state in List II.

Answer:

Q.5. State the correct term from A, B, C, D, E or F in List II which represents the change of state of matter or its relevant property from List I.

Answer:

New Simplified Chemistry Class 6 ICSE Solutions – Elements, Compounds & Mixtures

ICSE SolutionsSelina ICSE SolutionsML Aggarwal Solutions

Simplified ChemistryChemistryPhysicsBiologyMathsGeographyHistory & Civics

Exercise
Elements and Compounds

Question 1.
Classify substances into pure and impure substances in the form of a chart or tabulation.
Answer:

Question 2.
Differentiate between the terms – elements, compounds & mixtures.
Answer:
Element is a pure substance.

1. It is the basic unit of matter and cannot be broken down into two or more simpler substances by any means.
2. It is mainly classified into metals, non-metals, metalloids and noble gases.

Compound is a pure substance.

1. It is formed by combination of two or more elements.
2. The elements are combined together in a fixed ratio.
3. It can be broken down into its elements by chemical means.

Mixture is an impure substance.

1. It is formed by combination of two or more elements, compounds or both.
2. The substances are mechanically mixed together in any ratio.

Question 3.
The important physical properties of substances are colour, odour, nature, density & solubility in water. Name –
(a) two coloured gases (with their colours)
(b) a gas with a pungent, choking odour which is lighter than air
(c) a poisonous gas almost as heavy as air.
Answer:
(a)

1. Chlorine – greenish yellow
2. Nitrogen dioxide – reddish brown.

(b) Ammonia.
(c) Carbon monoxide

Question 4.
Complete the statement – an element is a pure substance made up of (identical/different) atoms.
Answer:
An element is a pure substance made up of identical atoms.

Question 5.
Draw a labelled diagram of an atom including its nucleus, orbits & their contents.
Answer:

Question 6.
Elements are classified into – Metals – Non-metals – Metalloids – Noble gases. State which of A, B, C, D is a –

1. Metallic element
2. Non-metallic element
3. Metalloid
4. Noble gas.

A : Is non-malleable, non-ductile & a poor conductor of electricity____
B : Has lustre, is malleable and ductile & a good conductor of heat ____
C : Is unreactive and inert and present in traces in air____
D : Shows properties of both metals and non-metals____
Answer:
A : Is non-malleable, non-ductile & a poor conductor of electricity Non-metallic element
B : Has lustre, is malleable and ductile & a good conductor of heat Metallic element
C : Is unreactive and inert and present in traces in air Noble gas.
D : Shows properties of both metals and non-metals Metalloid

Question 7.
An atom of an element is denoted by a ‘symbol’. Explain the meaning of the term “symbol” State a reason for representing the following elements by their symbols.

(a) Hydrogen by ‘H’
(b) Helium by ‘He’
(c) Copper by ‘Cu’

Answer:
Symbols : The short form or abbreviated name of the element (or radicals)
The reason for representing the following elements by their symbols are as follow :

(a) Hydrogen by ‘H’ – First letter of the name of element
(b) Helium by He – First two letters of the name of Noble gases.
(c) Copper by ‘Cu’ – Deriving symbols from their Latin names

Question 8.
Match the metallic elements – with their correct symbols Metallic elements (a) Potassium (b) Sodium (c) Calcium (d) Magnesium (e) Zinc
(f) Aluminium (g) Iron (h) Lead (i) Copper (j) Mercury (k) Silver (l) Platinum (m) Gold
Symbols : (1) Ca ; (2) Zn ; (3) Pb ; (4) Hg ; (5) Cu ; (6) Au ; (7) K ; (8) Fe ; (9) Al ; (10) Na ; (11) Mg ; (12) Pt ; (13) Ag
Answer:

Question 9.
Match the non-metallic elements – with their correct symbols
Non-metallic elements :
(a) Carbon (b) Chlorine (c) Oxygen (d) Phosphorus (e) Hydrogen (f) Nitrogen (g) Iodine (h) Bromine (i) Fluorine (j) Silicon (k) Sulphur
Symbols : (1) O (2) 1 (3) Si (4) C (5) Cl (6) P (7) F (8) H (9) S (10) Br (11) N
Answer:

Question 10.
Match the noble gases – with their correct symbols Noble gases : (a) Helium (b) Neon (c) Argon (d) Krypton (e) Xenon (f) Radon
Symbols : (1) Ar (2) Xe (3) Rn (4) He (5) Kr (6) Ne
Answer:

Question 11.
Give a reason why elements are tabulated in a table called the ‘Periodic table’.
Answer:
For arranging all the elements in a systematic and simple manner. The arrangement of elements was done in the form of a table called Periodic Table in which elements are arranged in increasing order of their atomic numbers. Elements in the Periodic Table are arranged in Horizontal rows called Periods and vertical columns called Groups. Metallic elements are placed on the left non-metallic on the right and noble gases on the extreme right of the Periodic table.

Question 12.
Give the names and symbols of the first twenty elements of the periodic table. Differentiate them into metals, non-metals, metalloids and noble gases.
Answer:

Question 13.
Explain the term – molecules. Give three examples of atoms of the same element forming a molecule. State the atomicity of the same.
Answer:
Atoms of the same element or different elements combine to form a molecule.
Atoms of the same element forming a molecule

1. Oxygen
2. Nitrogen
3. Hydrogen

Atomicity is the number of atoms present in one molecule of the element.

Question 14.
Give one example of
(a) a triatomic molecule
(b) a polyatomic molecule.
Answer:
(a) a triatomic molecule

(b) a polyatomic molecule.

Question 15.
Explain the term compounds. Give the example of a compound containing

(a) hydrogen and oxygen
(b)carbon and oxygen
(c) nitrogen and oxygen
(d) calcium and oxygen.

Answer:
A compound is a pure substance made up of two or more different elements combined chemically in a fixed proportion.
Example of Atoms of different elements forming a compound

(a) 2 Atom of H and 1 atom of O = H₂O
(b) Carbon dioxide, 1 atom of carbon and 2 atom of O = CO₂
(c) Nitrogen dioxide, 1 atom of nitrogen 2 atom of O = NO₂
(d) Calcium oxide, 1 atom of calcium 1 atom of O = CaO

Question 16.
State two characteristics of water which prove that it is a – compound.
Answer:
Characteristics of compounds

1. Elements in a compound are present in a definite proportion.
   Example 2 atoms of hydrogen combine with 1 atom of oxygen to give 1 molecule of water (compound)
   H₂ + O = H₂O (water)
2. Compounds have a definite set of properties example :
   The properties of the compound water are different from the properties of the elements hydrogen and oxygen in water.

Question 17.
Explain the terms ‘chemical formula\ State what a chemical formula denotes.
Answer:

• Chemical formula is a representation of a substance either element or compound by means of symbols.
• Chemical formula denotes the number of atoms of each element present in a compound.

Question 18.
Give the symbols and the number of atoms of each element present in

(a) sodium chloride
(b) water
(c) carbon dioxide
(d) zinc chloride.

Answer:

Question 19.
For writing a chemical formula – ‘symbols’ and combining capacity of an element with hydrogen i.e. ‘valency’ should be known. Explain the term – combining capacity of an element i.e. valency.
Answer:
The combining power of an element especially as measured by the number of hydrogen atoms can combine or displace with the element.

Question 20.
State what are radicals. Give the names of the radicals –

(a) NO₃
(b) OH
(c) SO₄
(d) CO₃

Answer:
A radical is an atom or group of atom of same or different elements that behaves in the manner of positive or negative ions.

(a) NO₃ – nitrate
(b) OH – hydroxide
(c) SO₄ – sulphate
(d) CO₃ – carbonate

Question 21.
Match the symbols of metallic elements – with their correct combining power or capacity.

(a) K
(b) Zn
(c) Al
(d) Na
(e) Ca

Combining power or capacity – A : 3 ; B : 2 ; C : 1. (positive valencies)
Answer:

(a) K — Valency-1 (C)
(b) Zn — Valency-2 (B)
(c) Al — Valency-3 (A)
(d) Na— Valency-1 (C)
(e) Ca — Valency-2 (B)

Question 22.
Match the symbols of non-metallic elements with their correct combining power or capacity.

(a) O
(b) S
(c) Cl

Combining power or capacity A : 3; B : 2; C : 1. (negative valencies)
Answer:

(a) O — Valency-2 (B)
(b) S — Valency-2 (B)
(c) Cl — Valency-1 (C)

Question 23.
Match the symbols of – radicals – with their correct combining power or capacity, (valency).

(a) OH
(b) SO₄
(c) NO₃
(d) CO₃

Combining power or capacity – A : 3; B : 2; C : 1. (negative valencies)
Answer:

(a) OH— Valency- 1 (C)
(b) SO₄ — Valency- 2 (B)
(c) NO₃— Valency- 1 (C)
(d) CO₃ — Valency- 2 (B)

Question 24.
Write the formula of the compound formed – given “symbols and combining power or capacity (valency) of each element in the compound.

(a) K¹⁺ Cl^1-
(b) Na¹⁺ Cl^1-
(c) Ca²⁺ NO₃^1-

Answer:

Question 25.
Match the formulas of the following – gases – with their correct names :
Gases : (a) H₂, (b) N₂, (c) O₂, (d) Cl₂, (e) HCl, (f) NH₃, (g) CO, (h) CO₂, (i) SO₂, (j) NO, (k) NO₂
Names :
(1) Ammonia, (2) Nitrogen dioxide, (3) Oxygen, (4) Hydrogen, (5) Nitrogen, (6) Chlorine, (7) Carbon monoxide, (8) Sulphur dioxide, (9) Nitrogen monoxide, (10) Carbon dioxide, (11) Hydrogen chloride
Answer:

Question 26.
Match the formulas of the following – acids – with their correct names
Acids : (a) HCl, (b) HNO₃, (c) H₂SO₄, (d) H₂CO₃
Names : (1) Carbonic acid, (2) Sulphuric acid, (3) Hydrochloric acid, (4) Nitric acid
Answer:

Question 27.
Match the formulas of the following – bases – with their correct names.
Bases : (a) NaOH, (b) KOH, (c) Ca(OH)₂, (d) Zn(OH)₂
Names : (1) Potassium hydroxide, (2) Zinc hydroxide, (3) Aluminium hydroxide, (4) Calcium hydroxide, (5) Sodium hydroxide
Answer:

Question 28.
Complete the statements with the correct words.

(a) Acid’s – are chemicals which are ____ in taste and derived from plants and ____
Answer:
Acids – are chemicals which are sour in taste and derived from plants and minerals.

(b) Bases – are chemicals which are hydroxide (or oxides) of ____ eg. sodium hydroxide.
Answer:
Bases – are chemicals which are hydroxide (or oxides) of metals eg. sodium hydroxide.

(c) Salts- are chemicals formed on reaction of a base with an ____ giving salt and water.
Answer:
Salts- are chemicals formed on reaction of a base with an acid giving salt and water.

Question 29.
In the chemical word equation – Zinc + Sulphuric acid → Zinc sulphate + Hydrogen

(a) State the reactants and products of the above reaction. What does the arrow indicate.
Answer:

(b) The molecular equation is :
Zn + H₂SO₄ → ZnSO₄ + H₂[g]. State what [g] represents.
Answer:
[g] represents gas.

Objective Type Questions

Q.1. Complete the statements given below by filling in the blank with the correct word/s.

1. An element is a pure substance which cannot be broken down by physical or chemical methods.
2. The basic unit of an element is an atom.
3. Atom contains nucleus, with positively charged protons.
4. Element silver has the symbol derived from its Latin name ‘argentum’.
5. From the elements – He, Br, Pt and O ; the element which is metallic is Pt, an inert element is He, forms a triatomic molecule is O, is liquid at room temperature is Br.
6. From the elements nitrogen, chlorine, bromine, the element present in the atmosphere is nitrogen.

Q.2. Match the statements in List I – 1 to 10 with their correct answers in List II – A to J.

Answer:

Q.3. Select the correct answer from the choice in brackets.

Question 1.
The symbol of – mercury [Mg / Hg / Ag]
Answer:
Hg

Question 2.
The type of element-phosphorus [metallic/non-metallic/noble gas]
Answer:
non-metallic

Question 3.
The type of molecule- bromine [monoatomic/diatomic/triatomic]
Answer:
diatomic

Question 4.
A compound [nitrogen/ozone/zinc chloride]
Answer:
zinc chloride

Question 5.
The unreactive non-metallic element [chlorine / argon / sulphur]
Answer:
argon

Question 6.
The negatively charged particles in an atom [protons/neutrons/electrons]
Answer:
electrons

Question 7.
The element which is malleable [sulphur / carbon / iron]
Answer:
iron

Question 8.
An impure substance [element / mixture / compound]
Answer:
mixture

Question 9.
An element which is a poor conductor of heat [copper / carbon / aluminium]
Answer:
carbon

Question 10.
A group of atoms of elements [ion / radical / combining power]
Answer:
radical

Q.4. Match the compounds in List I – 1 to 20 with their correct formulas from in List II – A to T.
1. Hydrochloric acid, 2. Potassium hydroxide, 3. Sulphuric acid, 4. Zinc hydroxide, 5. Sodium chloride, 6. Nitric acid, 7. Calcium hydroxide, 8. Carbonic acid, 9. Sodium hydroxide, 10. Copper sulphate, 11. Potassium chloride, 12. Calcium carbonate, 13. Magnesium suiphide, 14. Zinc sulphate, 15. Aluminium hydroxide, 16. Sodium carbonate, 17. Calcium sulphide, 18. Magncsium oxide, 19. Zinc oxide, 20. Ammonium chloride.

Answer:

Exercise
Mixtures

Question 1.
Explain the term mixtures. Give an example of mixtures of –

(a) two elements
(b) two compounds
(c) elements and compounds.

Answer:
Mixture : A mixture is an impure substance made up of two or more elements or compounds mechanically mixed together in any proportion.
Examples :
(a) Of two elements

(b) Of two compounds

(c) Of elements and compounds

Question 2.
Differentiate between homogeneous and heterogeneous mixtures with suitable examples.
Answer:
The main characteristics of mixture (Iron + Sulphur) are :

Homogeneous mixtures :

1. There constituents are uniformly mixed.
2. The properties and composition are same throughout the mixture.
3. Examples :
   (a) alcohol and water
   (b) salt and water
   (c) sugar and water

Heterogeneous mixtures :

1. There constituents are not uniformly mixed.
2. The properties and composition vary throughout the mixture.
3. Examples :
   (a) oil and water
   (b) sand and water
   (c) chalk and water

Question 3.
With reference to the mixture of iron and sulphur, state the main characteristics of mixture.
Answer:
The main characteristics of mixture (Iron + Sulphur) are :

1. The mixture of iron and sulphur may contain iron and sulphur in varying proportions.
2. The original properties of each element are retained in the mixture of iron and sulphur.
3. Mixture of the two elements iron and sulphur can be separated by i.e. a physical method, i.e. by using a magnet since iron is attracted to the magnet.

Question 4.
Tabulate a comparative chart – to differentiate between elements, compounds and mixtures. Differentiate them with reference to

(a) the term
(b) existence
(c) properties
(d) separation of components.

Answer:
(a) Term —

• Elements – Pure substance made up of one kind of atoms only. e.g. Iron [Fe], Sulphur [S]
• Compounds – Pure substance made up of two or more different elements, e.g. Iron sulphide [FeS]
• Mixtures – Impure substance made up of two or more elements or compounds, e.g. Iron and sulphur mixture.

(b) Existence—

• Elements – Elements i.e. atoms are present on their own. e.g. Iron and sulphur exist on their own as elements iron and sulphur.
• Compounds – Components in a compound present in a definite proportion.
  e.g. Iron and sulphur are chemically combined in a fixed ratio in iron sulphide.
• Mixtures – Components in a mixture present in any proportion.
  e.g. Iron and sulphur are mixed in any ratio in the mixture of iron and sulphur.

(c) Properties —

• Elements – Elements have a definite set of properties.
  Elements classified into metal and non-metals each with its own properties.
• Compounds – Compounds have a definite set of properties. Elements of a compound do not retain their original properties.
• Mixtures – Mixture not have a definite set of properties.
  Components of a mixture do retain their original properties.

(d) Separation of compound —

• Elements – Elements occur on their own or as compounds and can be separated by chemical and physical methods.
  Example : Iron, copper.
• Compounds – Elements in a compound are chemically combined and can be separated by chemical methods only.
  Example : Iron Sulphide, copper oxide.
• Mixtures – Components in a mixture can be separated by , physical methods only.
  Example : Iron + sulphur

Question 5.
State the principle involved in separation of solid-solid mixtures by –

(a) sieving
(b) magnetic separation
(c) sublimation.

Answer:
(a) Sieving

• Principle — Based on the difference in size of the solid particles.
• Examples — Mixture of Rice powder and soil, mixture of different sized particles of diamond and of sand.

(b) Magnetic separation

• Principle — One component of the mixture is a magnetic substance.
• Example — Mixture of iron ore and sand, Mixture of cobalt and lead.

(c) Sublimation

• Principle — One of the components sublimes on heating.
• Example — Mixture of iodine and salt, Mixture of ammounium chloride and sand.

Question 6.
State the principle involved in separation of solid-liquid mixtures by –

(a) sedimentation and decantation
(b) filtration
(c) evaporation

Answer:
(a) Sedimentation and decantation —

• Principle — The solid component is insoluble and heavier than the liquid component.
• Example — Mixture of sand and water

(b) Filtration —

• Principle — Seperation of insoluble solid component by “ passing through a porous material like filter paper.
• Example — Mixture of chalk and water, Mixture of Agcl and Water.

(c) Evaporation —

• Principle — Seperation of the mixture by evaporating the liquid component. The solid should be soluble in the liquid and should not sublime.

Question 7.
Explain the term ‘sieving’. State the structure of a sieve and explain the separation of different sized particles by sieving.
Answer:
Sieving is a method of separation, which is used for separating substances of different sizes that cannot be separated in hand picking.
Principle — Based on the difference in size of the solid particles.

• Technique of Separation — The large sized par icles are separated from the small or finer particles by passing the mixture through a sieve.
  The sieve has a wooden frame, with a metal mesh at its base.
  The mixture is added from the top of the sieve, when the larger particles stay above and the finer particles collect below it on shaking the sieve.
• Examples — Separation of rice powder from soil, seperation of different sized particles of diamond and of sand.

Question 8.
State what is meant by ‘magnetic separation of two mixtures’. Explain how iron particles can be separated from sulphur particles.
Answer:
Magnetism is ideal for separating mixtures of two solids with one part having magnetic properties. Some metals like iron, nickel and cobalt have magnetic properties whiles gold, silver and aluminum do not. Magnetic elements are attracted to a magnet.

• Principle — Based on the difference in magnetic and non-magnetic nature of particles.
  
• Technique of Separation — The magnetic particles such as iron are separated from the non-magnetic particles such as sulphur – by utilizing the magnetic properties of iron. The iron gets attracted to the magnet and seperates from the non-magnetic substance.

Question 9.
Give a reason why sublimable and non-sublimable substances can be separated easily, but two sublimable substance cannot.
Answer:
While seperating the mixture sublimable and non- sublimable substances, sublimable substance turns directly into vapour on heating whereas non-sublimable solid remains behind. Since, if we try to seperate the mixture of two sublimable substances, then both the sublimable substances turns into vapour on heating and vapour on cooling give back the pure same solid.

Question 10.
Explain the technique for separating – insoluble solid particles in a solid-liquid mixture.
Answer:
The insoluble solid particles in a solid-liquid mixture can be seperated by filtration.
Technique of Seperation — A filter paper is made into a cone & placed in a funnel. The solid particles remain behind on the filter paper while the liquid collects below.

Question 11.
Differentiate between the terms sedimentation and decantation with a suitable experimental technique.
Answer:
Sedimentation and decantation are two very different terms. Sedimentation is defined as the process of settling down of the heavier components present in a mixture.
For example : When the mixture of sand in water is allowed to stand undisturbed for some time, it is observed that sand settles at the bottom.
Decantation is defined as the process of separating the Liquid portion of a mixture when the heavier component settles at the bottom as sediments.
In other words, it is the process of transferring a liquid from one container to another without disturbing the sediments that are present at its bottom.
For example : When a mixture of sand and water is allowed to stand, sand settles at the bottom of a container since it is heavier. Water is present in the upper portion of the container. This can be separated from sand settled at the bottom simply by pouring in a different container without using any other separating device. This is known as decantation.

Question 12.
Explain how a solid component is separated in a soluble solid-liquid mixture.
Answer:
A Solid component is separated in a soluble solid-liquid mixture by evaporation.
Principle — Seperation of the mixture by evaporating the liquid component. The solid should be soluble in the liquid and should not sublime.
Technique of Separation : The soluble solid can be seperated from its liquid component by allowing the liquid component to evaporate either on its own or by heating. During evaporation, the liquid component is lost to the atmosphere & the solid remains behind.
Example : Evaporation of a common salt solution or sea water leaves behind common salt.
Common salt remains behind whereas water lost to the atmoshphere.

Question 13.
Draw a neat labelled diagram for separation of the following mixtures.

(a) Coarse sand from fine sand
(b) A magnetic particle from a non-magnetic particle
(c) Naphthalene from sodium chloride
(d) Chalk and water using a filter paper
(e) Sand and water without using a filter paper
(f) Common salt from a solution of common salt and water

Answer:
(a) Coarse sand from fine sand

Separation of particles of sand, Coarse sand stays above and fine sand below.
(b) A magnetic particle from a non-magnetic particle

(c) Naphthalene from sodium chloride

(d) Chalk and water using a filter paper
Filtration of chalk particles in water.
Filtered out solid chalk particles.

(e) Sand and water without using a filter paper

(f) Common salt from a solution of common salt and water

Common salt remains behind.

Question 14.
State the technique involved in separating the following:

(a) Iodine crystals and potassium chloride
(b) Iron and chalk powder
(c) Potassium chloride from an aqueous solution of potassium chloride.
(d) Rice powder from soil particles
(e) Iron filings from pieces of copper wire
(f) Large diamonds from very small diamonds

Answer:

(a) Sublimation
(b) Magnetic separation
(c) Evaporation
(d) Seiving
(e) Magnetic separation
(f) Seiving

Objective Type Questions

Q.1. Match the statements in List I with the correct answer in List II.

Answer:

Q.2. State whether the following statements are true or false. If false write – the correct statement.

1. Components in a mixture are present in a definite proportion.
Ans. False.
Correct — Components in a mixture are present in a varying proportion.

2. Mixture have no definite set of properties.
Ans. True.

3. Components in a mixture can be separated by physical and chemical methods.
Ans. False.
Correct — Components in a mixture can be separated by physical methods.

4. Sodium chloride and water is an example of a mixture of elements and compounds.
Ans. False.
Correct — Sodium chloride and water is an example of a mixture of compounds.

5. Heterogeneous mixtures have different composition and properties throughout the mixture.
Ans. True.

Q.3. Name the following :

Question 1.
A black non-metallic component of the mixture gunpowder.
Answer:
Carbon.

Question 2.
A physical method of separating iron from an iron- sulphur mixture.
Answer:
Magnetic separation.

Question 3.
The component of the soluble solid-liquid mixture, which is lost to the atmosphere on heating.
Answer:
Liquid component i.e. water.

Question 4.
The component of a naphthalene-sodium chloride mixture, which remains in the evaporating dish when heated together, covered with a funnel.
Answer:
Sodium chloride.

Question 5.
A mixture of two immiscible liquids.
Answer:
Kerosene oil and water.

Q.4. Give reasons for the following :

Question 1.
Naphthalene and camphor cannot be separated by sublimation.
Answer:
Because both of these are sublimable solids and they sublimes on heating.

Question 2.
Sodium chloride cannot be separated out from its aqueous solution by filtration.
Answer:
Filtration is a method that is used to separate insoluble solids from liquids, since both salt dissolve in water.

Question 3.
Two varieties of wheat flour cannot be separated by sieving.
Answer:
Since in sieving we can separate larger particle but in this case both particles are finer in nature.

Question 4.
Sulphur and charcoal powder cannot be separated by magnetic separation.
Answer:
Because both are non-magnetic particles.

Question 5.
Both components of a soluble solid-liquid mixture cannot be recovered by evaporation.
Answer:
Because liquid is lost/evaporated in atmosphere.

New Simplified Chemistry Class 6 ICSE Solutions – Introduction to Chemistry

ICSE SolutionsSelina ICSE SolutionsML Aggarwal Solutions

Simplified ChemistryChemistryPhysicsBiologyMathsGeographyHistory & Civics

Exercise

Question 1.
State what is Science and give the basic bifurcation of Science with reasons.
Answer:
Science is the subject which includes the study of various experiments performed by a scientist with all observations recorded and inferences concluded.
Bifurcation : Science is bifurcated into three main branches— Physics, Chemistry and Biology which make study of each branch simple and more focussed.

Question 2.
Give the basic difference between Inorganic Chemistry and Organic Chemistry.
Answer:
(a) Inorganic chemistry includes study of innumerable elements and compounds

(b) Organic chemistry includes study of specific carbon compounds built up mainly of carbon and hydrogen.

• Compounds include hydrocarbons e.g., Methane
• Acids include-acetic acid

Question 3.
State the functions of the following basic glass apparatus.

(a) Glass test tube
(b) Hard glass boiling tube
(c) Beaker
(d) Round bottom flask
(e) Retort

Answer:

(a) Glass test tube : It is used for heating chemicals and studying reactions in chemicals solution.
(b) Hard glass, boiling tube : It is resistant to chemicals and used for special purposes. It is made of pyrex.
(c) Beaker : It is used for holding pouring and mixing solutions.
(d) Round bottom flask : In preparation of gases, where heating is required. Since the flask is round bottomed, heat is uniformly distributed throughout on heating.
(e) Retort : For carrying out distillation experiments which include distillation of acids.

Question 4.
State why a round bottom flask is preferred to a flat bottom flask – during preparation of gases in the laboratory.
Answer:
Round bottom flask is preferred to flat bottom flask because in round bottom flask heat is uniformly distributed throughout on heating.

Question 5.
State the function of –

(a) The wooden handle at the end of the test tube holder.
(b) The asbestos fixed at the centre of the wire gauze.
(c) The clamp mounted on the rod of a retort stand.
(d) The air regulator in a Bunsen burner.
(e) The tap at the base of a burette.

Answer:

(a) The wooden handle at the end of test tube being a poor conductor of heat, makes holding the test tube holder easy.
(b) The asbestos fixed at the centre of the wire gauze initiates even distribution of heat to the bottom of the apparatus. It also prevents glass apparatus from cracking.
(c) The clamp mounted on the rod of a retort stand helps in holding the apparatus and adjusting its level upwards or downwards.
(d) The air regulator has holes and is used for regulating the flame.
(e) The tap at the base of the burette helps in removing the measured liquid drop wise.

Question 6.
State the method used for collecting the following gases –

(a) Oxygen – a gas slightly soluble in water.
(b) Ammonia – a gas highly soluble in water and lighter than air.
(c) Sulphur dioxide – a gas highly soluble in water and heavier than air.

Answer:

(a) The method used for collecting oxygen gas is downward displacement of water.
In this method, the gas jar is filled with water and inverted over the beehive shelf, so that the water is displaced downwards and oxygen is collected upwards.
(b) The method used for collecting ammonia gas is downward displacement of air.
This method is used for gases like ammonia which is soluble in water and lighter than air.
(c) The method used for collecting SO₂ is upward displacement of air.
This method is used for gases like SO₂ which is soluble in water and heavier than air.

Question 7.
Explain in brief the importance of Chemistry in agriculture and production of various products.
Answer:
Importance of Chemistry : In agriculture end products and as follow :
1. AGRICULTURE
(a) Fertilizers – A substance to improve fertility and supply of nutrients to plant and is essential for growth.
Example :

1. Ammonium nitrate – widely used as a fertilizer.
2. Urea – an important source of nitrogen [non-explosive and solid in nature]
3. Phosphatic fertilizer – super phosphates.

About 90% of fertilizers are in the solid form. Liquid fertilizers comprise – aqueous solutions of amnionia or ammonium nitrate.

(b) Pesticides

1. Chemicals added to the soil, to kill pests. They include – Herbicides; insecticides; termiticides etc.
2. Pesticides protect the plants from – weeds, fungi and insects.
3. About 30% of crops are destroyed by agricultural pests.

(a) Herbicides – Kill or inhibit growth of – unwanted plants.
(b) Insecticides – Destroy insects, which harm or destroy plants.

Pesticides may come in contact with other living organisms and disrupt the balance of the eco-system.

2. PRODUCTS
Knowledge of Chemistry has initiated production of – different products

1. Food – Refined oils, butter, cheese, etc. are obtained through chemical reactions.
2. Construction – Mortar, cement, glass, etc. are various chemical compounds.
3. Clothing – Natural fabrics such as silk are made through chemical reactions.
4. Household – Cooking gas which is liquefied petroleum gas or LPG, food preservatives, specific utensils and electronic items, all involve use of Chemistry.
5. Daily usage – Paints, dyes, perfumes, paper, ink etc. involve chemical reactions.
6. Industrial – Metals & alloys [mixture of metals] which find application in innumerable machines and metallic structures like automobiles, involve Chemistry.
7. Petrolium – Petrol, kerosene & diesel oil are products – obtained from petroleum

Question 8.
‘Alchemists are older words for Chemists’. Discuss the statement in brief.
Answer:
Alchemy and alchemists are older words for Chemistry and chemists where – alchemists transformed or created substances through a seemingly – magical process.
An alchemists Nicolas flamel claimed to transform metals into gold.

Question 9.
In the medieval ages – philosopher’s stone was connected with all transformations. Explain.
Answer:
In the medievel ages to the 17th Century the so-called philosopher’s stone – held priority and alchemists were successful to a certain extent, in the development of processes, which helped later chemists to extract metals and develop – path-breaking avenues in Chemistry. The scientific process involving modern chemistry started poving paths and chemistry regains its rightful position. The Scientific Process involving Modern Chemistry started paving paths & chemistry regains its rightful position.

Question 10.
State the contributions of

(a) Dimitri Mendeleev
(b) Antoine Lavoisier
(c) John Dalton – towards the development of Chemistry.

Answer:
(a) Dimitri Mendeleev : He formulated the – Periodic Table of elements.

1. He systematically arranged the dozens of known element by atomic weights and could even predict the properties of the still unknown elements.
2. He devised the Periodic Table. He was best known for his discovery of the – Periodic Law.

(b) Antoine Lavoisier :

1. He recognised and named oxygen in 1778 & later – hydrogen in 1783.
2. He also wrote the first extensive list of elements and helped to reform chemical nomenclature.
3. In 1774, he turned his attention to the phenomenon of combustion with his famous experiment, in which he heated pure mercury in a swan necked retort, leading to the discovery of oxygen.

(c) John Dalton :

1. He compiled his theory in 1803 known as Dalton’s atomic theory.
2. The main postulates of Dalton’s atomic theory are that “Matter consists of particles called atoms, which are indivisible and cannot be created or destroyed.”
3. The theory was later contradicted in certain aspects by the Modem atomic theory.

Question 11.
Differentiate between the terms – food preservatives and food processing with appropriate examples.
Answer:
Food preservatives – are substances or chemicals – added to food or beverages to

1. Prevent decomposition by bacteria or microbes.
2. Reduce risk of food borne infections.
3. Preserve nutritional quality of food.

Preservatives :

1. Benzoic acid
2. Nitrates
3. Sulphur compounds

Food items :

1. Jams, pickles, carbonated drinks.
2. Meat products
3. Beverages, wines etc.

Question 12.
Explain the term ‘cosmetics’ & ‘talc’. Name a few main ingredients in cosmetics the role they play.
Answer:
Cosmetics are mixtures of chemical compounds from natural sources or from synthetic sources.
Cosmetics – enhance or alter the appearance or fragrance of an individual. Some sources of compounds used in cosmetics include – modified natural oils and fats, processed minerals e.g. – zinc oxide, iron oxide and talc.
Talc :

1. Talcum powder is made from talc – a mineral made up of – hydrated magnesium silicate, (contains elements – magnesium silicon, oxygen).
2. In its natural form – talc contains asbestos – which is removed from consumer products.
3. Talc – absorbs moisture, cuts down on friction, keeps skin dry and prevents rashes.

Question 13.
Differentiate between – natural fibres & synthetic fibres. State what is – Terylene. State some of its characteristic properties which make it suitable for commercial use.
Answer:
Natural fibres – such as cotton and wool which were directly converted into clothing material.
Synthetic fibres – such as terylene, nylon and rayon are used in expensive clothing ; carpets etc.
Terylene – It is a synthetic polyester fibre or fabric formed generally, by – addition of polyester to natural fibre – cotton. The combination makes the fabric, easy to clean and crease resistant.,

Properties – It is a strong fabric –

(a) elastic in nature, resistant to friction,
(b) suffers little loss in strength,
(c) crease resistant,
(d) easily washable and dries quickly.

Uses –

(a) in fashion garment fabrics
(b) in nonwoven carpets, rain coats, sails
(c) in making of nets, ropes, hoses etc.

Question 14.
Explain how medicines are a boon to mankind. State the positive & negative effects of medicines like aspirin and paracetamol.
Answer:
Medicines are natural or synthetic substances which when taken in a living body, affects its functioning, and treats or prevents a disease.
Aspirin
Positive effect :

1. It is a medicine to treat pain, fever and inflammation.
2. Aspirin given shortly after a heart attack, may decrease risk of death.
3. As long term use it may reduce, blood clots in people who are at a high risk.

Negative effect :

1. It is generally not recommended in children with infections.
2. Its side effects may include upset stomach, stomach ulcers etc.

Paracetamol :

1. It is a medicine to treat mild to moderate pain and fever.
2. It may also be used in low back pain, headaches and for dental use.

Negative effect :

1. It maybe sold in combination with cold medications.
2. It is safe at recommended doses, but too high a dose may result in liver problems.

Question 15.
Give a comparitive difference between – soaps and detergents. Give a reason why detergents have an advantage over soap.
Answer:
Soaps are substances used with water, for cleaning and washing and are made from a compound of vegetable oils or animal fats along, with sodium or potassium hydroxide and generally have perfumes or colourants, added to it.
Whereas
Detergents are synthetic water soluble cleaning agents that unlike soap (which are prepared from vegetable oils or fats) are prepared from petroleum products along with sodium or potassium hydroxide.
Detergents have an advantage over ordinary soap

1. Hard water is one which does not lather with soap, while soft water lathers easily.
2. Ordinary soap when rubbed in hard water is wasted and lather forms only after all the insoluble salts in hard water are removed as scum.
3. Synthetic detergents do not form scum and lather even in hard water.

Question 16.
State what are – ‘stain removals’. Name three important stain removers & give their prime functions.
Answer:
Stain removals It is the process of removing a mark or a stain left by one substance on a specific surface fabric. Most stains are removed by dissolving with a solvent.
Examples of stain removers

Objective Type Questions

Q.1. Select the correct name from the choice A, B or C in each case.

Question 1.
The branch of Science which deals with the different forms of energy e.g. light and sound.
A : Chemistry
B : Physics
C : Biology
Answer:
B : Physics

Question 2.
The branch of Chemistry which includes study of specific carbon compounds – built up of mainly carbon and hydrogen.
A : Inorganic Chemistry
B : Physical Chemistry
C : Organic Chemistry
Answer:
C : Organic Chemistry

Question 3.
The scientist who formulated the Periodic Table.
A : John Dalton
B : Daniel Rutherford
C : Demitri Mendeleev
Answer:
C : Demitri Mendeleev

Question 4.
Predecessors to the modern Chemist who created the Philosoper’s stone’.
A : Botanists
B : Alchemists
C : Physicists
Answer:
B : Alchemists

Question 5.
A synthetic fibre used in clothing.
A : Cotton
B : Jute
C : Terylene
Answer:
C : Terylene

Question 6.
A medicine to treat pain, fever & inflammation.
A : Anaesthetic
B : Aspirin
C : Antacid
Answer:
B : Aspirin

Question 7.
A water soluble cleaning agent – not inactivated by hard water.
A : Soap
B : Detergent
C : Cleanser
Answer:
B : Detergent

Question 8.
A substance which absorbs moisture and keeps the skin dry and free from rashes.
A : Talc
B : Preservative
C : Emulsifier
Answer:
A : Talc

Question 9.
A chemical used as a preservative for jams, pickles etc.
A : Titanium dioxide
B : Rayon
C : Benzoic acid
Answer:
C : Benzoic acid

Question 10.
A substance which dissolves grease & oil and is preferred as a – drain cleaner.
A : Glycerine
B : Sodium hydroxide
C : Hydrogen peroxide
Answer:
B : Sodium hydroxide

Q.2. Match the apparatus A to J shown below with their correct names 1 to 10 given below :



Answer:

Q.3. Select the correct apparatus from A, B or C for each of the functions of the apparatus given.

Question 1.
For gas preparations where heating is required.

(A) Flat bottom flask
(B) Retort
(C) Round bottom flask

Answer:
(C) Round bottom flask

Question 2.
For holding washed test tubes.

(A) Test tube holder
(B) Test tube stand
(C) Retort stand

Answer:
(B) Test tube stand

Question 3.
A glass apparatus resistant to chemicals, made of pyrex and used for heating specific liquids.

(A) Beaker
(B) Retort
(C) Boiling Tube

Answer:
(C) Boiling Tube

Question 4.
A metallic apparatus which supports the wire gauze.

(A) A Tripod stand
(B) Retort stand
(C) Test tube stand

Answer:
(A) A Tripod stand

Question 5.
A long glass apparatus closed at one end used for collecting gases.

(A) Measuring cylinder
(B) Gas jar
(C) Beehive shelf

Answer:
(B) Gas jar

Question 6.
A glass apparatus which measures liquid by sucking the liquid at one end upto the marked level and later pouring it out.

(A) Burette
(B) Measuring cylinder
(C) Pipette

Answer:
(C) Pipette

Question 7.
A long glass tube with a broad inlet at the top, which allows entry of the reactants into the round bottom flask, during laboratory preparations of gases.

(A) Thistle funnel
(B) Delivery tube
(C) Funnel.

Answer:
(A) Thistle funnel

Question 8.
A clay vessel kept in a trough of water during collection of a gas by downward displacement of water.

(A) Retort
(B) Beehive shelf
(C) Burette

Answer:
(B) Beehive shelf

Question 9.
A modern apparatus with an air regulator, used for heating purposes.

(A) Spirit lamp
(B) Bunsen burner
(C) Electric stove

Answer:
(B) Bunsen burner

Question 10.
A rectangular mesh with an asbestos at its centre, kept for initiating even distribution of heat to the bottom of the apparatus.

(A) Tripod stand
(B) Wire gauze
(C) Retort stand

Answer:
(B) Wire gauze

Q4. Give reasons for the following :

Question 1.
Alchemy was considered a pseudoscience.
Answer:
Towards the end of the 17th Century the scientific processes involving modem Chemistry started paving paths and Alchemy today is considered a pseudoscience and Chemistry regains its rightful position as a serious scientific field.

Question 2.
Preservatives are added to food or beverages.
Answer:
Preservatives are added to food or beverages because of following reasons :

1. Prevent decomposition by bacteria or microbes.
2. Reduce risk of food borne infections.
3. Preserve nutritional quality of food.

Question 3.
Titanium dioxide is an important ingredient in cosmetics.
Answer:
It is a natural pigment powder which provides a base for mineral makeup. It provides mild sun protection and as a pigment gives a white colouration to coloured ingredients.

Question 4.
Aspirin is one of the most widely used medication – globally.
Answer:
Aspirin is one of the most widely used medication – globally because of following reasons :

1. It is a medicine to treat pain, fever & inflammation.
2. Aspirin given shortly after a heart attack, may decrease risk of death.
3. As long term use it may reduce, blood clots in people who are at a high risk.

Question 5.
Ordinary soap is wasted in hard water.
Answer:
Ordinary soap is wasted in hard water because hard water is one which does not lather with soap.

Question 6.
A philosopher’s stone is not exactly a stone.
Answer:
Philosopher’s stone in a legendary substance, capable of turning inexpensive metals like lead or mercury into – gold.and silver.
(It was not literally a stone, but a powder or potion).

Question 7.
Food processing is an important procedure for obtaining marketable food products.
Answer:
Food processing – involves physical or phemical processes, to transform or change the raw ingredients in food into easy usable forms of food available in markets. Raw materials – in food to Marketable food products

Question 8.
Cosmetics may contain preservatives, as one of their ingredients.
Answer:
They extend the shelf life of a cosmetic and may prevent growth of microorganisms.

Question 9.
Polyester is added to natural fibre cotton, to give terylene.
Answer:
Polyester is added to natural fibre cotton, to give terylene because this combination makes the fabric easy to clean and crease resistant.

Question 10.
All medicines must be taken under proper doctors supervision and in the correct dose.
Answer:
All medicines must be taken under proper doctors supervision and in the correct dose because some medicine has side effect as aspirin not taken in proper dose may cause stomach ulcers similarly paracetamol if taken in high dose may cause liver problems.

New Simplified Chemistry Class 8 ICSE Solutions – Carbon & Its Compounds

ICSE SolutionsSelina ICSE SolutionsML Aggarwal Solutions

Simplified ChemistryChemistryPhysicsBiologyMathsGeographyHistory & Civics

EXERCISE

Question 1.
Differentiate between the two branches of chemistry – organic chemistry & inorganic chemistry with suitable examples.
Answer:

• Organic chemistry : “A branch of science that deals with the study of specific compounds (of carbon) hydrogen and oxygen.”
  e.g. methane CH₄, ethane C₂H₆, CH₃COOH, CH₄OH methyl alcohol.
• Inorganic chemsitry : “A branch of science that deals with the study of – oxides of carbon, carbonates, metallic carbides etc.”
  e.g. H₂C0₃ – carbonic acid, C0₂, CaC0₃ calcium carbonate.

Question 2.
State how carbon occurs in the free state and in the combined state.
Answer:

• Occurence of carbon : In free state – diamond, graphite, in impure form as coal.
• In combind state : C0₂ in atmosphere, hydro-carbon as natural gas, petroleum, cellulose in wood and limestone CaC0₃.

Question 3.
Define the term ‘allotropy’. Give a reason why carbon exhibits allotropy.
Answer:
Allotropy : “The phenomenon of occuring of an element in two or more forms having same chemical properties but different physical properties.”
Carbon shows allotropy as a result of – difference in atomic arrangement in the crystal structure.

Question 4.
Name two crystalline and four amorphous allotropes of carbon.
Answer:
Allotropes of carbon:

Crystalline :

1. diamond
2. graphite.

Amorphous :

1. coal,
2. coke
3. lampblack
4. wood charcoal.

Question 5.
Compare the structure of the crystal of diamond & graphite with special reference to the reason for diamond being the hardest natural substance while graphite one of the softest. Compare the electrical & thermal conductivity of the two crystalline allotropes of carbon.
Answer:

• Structure of crystal of diamond : Hardest natural substance is due to – strength and uniformity of the carbon – carbon covalent bond [tetrahadron structure].
  It is bad conductor of electricity and heat due to abscence of free electrons in the crystal, i.e. no electron is free to move.
• Structure of graphite : “Is soft and greasy due to parallel layers of C-atoms held together by weak VANDER WAAL – forces which slide over one another, i.e. hexagonal structure.
  It is good conductor of heat and electricity due to – presence of mobile electrons in the crystal.

Question 6.
With reference to the structure of the two crystalline allotropes of carbon, state why diamond is inert or unreactive while graphite is comparably more reactive.
Answer:
Structure of diamond is compact and hence unreactive. Graphie has open structure and more prone to chemical attack.

Question 7.
State the reasons for (a) Use of diamond – as an. item of jewellery, (b) Use of graphite – (i) as a lubricant for heated machine parts, (ii) as a lining for crucibles used in manufacture of high grade steel, (iii) as an electrode in electroplating.
Answer:
(a) Use of diamond – as an item of jewellery as – diamond has sparkling brilliance due to high refractive index is cut and polished.

(b)

1. Heavy machines which run very fast, when other lubricants are used catch fire, but graphite as lubricant when used parallel layers slide over one and other is non-volatile and non-sticky and reduces friction between rotating parts.
2. Graphite is used in refractory crucibles as graphite can withstand high temperature is a good conductor of heat.
3. Graphite as electrode in electroplating as graphite is good conductor of electricity relatively inert and almost does not react with acids.

Question 8.
State in brief the transformation of vegetable matter to different types of coal varying in carbon content. State two uses each of

(a) coal
(b) coke
(c) lampblack or soot.

Answer:
Transformation of vegetable matter to coal is due to slow bacterial decomposition of vegetable matter under the influence of heat from within the earth, high pressure of earth (soil) above it and in absence of oxygen over millions of year.
The different stages of transformation of vegetable matter results in different types of coal

1. 60%C is peat
2. 65%C is lignite
3. 85%C is bituminous
4. 90%C is anthracite

Question 9.
‘Wood charcoal – an amorphous aliotrope of carbon reduces heated metallic oxides to metals.’ Give a balanced equation to support the statement.
Answer:
Wood charcoal reduces ZnO, Fe2O₃ to metal.

Question 10.
Carbon dioxide occurs both in the free state and in the combined state.
State three methods how carbon dioxide is added to the atmosphere.
Answer:
Carbon dioxide is added to the atmosphere by

1. Burning of carbon compounds – wood petroleum.
2. Death and decay of organic matter.
3. Respiration in living organisms.

Question 11.
In the laboratory preparation of carbon dioxide by action of a dilute acid on a metallic carbonate give – (a) A balanced equation for the preparation (b) A reason for (i) use of a washer bottle containing cone. H₂SO₄ in the preparation (ii) Not collecting the prepared gas over water (iii) Not using dilute sulphuric acid as a reactant in the preparation.
Answer:

(b)

1. Cone. H₂SO₄ used in washer bottle absorbs moisture from the gas.
2. C0₂ is soluble in water and hence is not collected over water.
3. Dil. sulphuric acid is not used as reactant. Since H₂SO₄ reacts with calcium carbonate forming- a coating of insoluble calcium sulphate [CaSO₄]. The reaction slowly comes to a stop.
   

Question 12.
How would you prove experimentally that

(a) Carbon dioxide does not support combustion
(b) Is slightly acidic in nature.

Answer:
(a) Carbon dioxide does not support combustion.

• Experiment : Bring a burning candle in a gas jar full of CO₂ gas. The candle is extinguished. This shows that CO₂ is non-supporter of combustion.
  

(b) CO₂ gas is acidic in nature. Bring a moist blue; litmus paper in the mouth of gas jar full of CO₂ gas. The blue colour of filter paper changes red. This shows that CO₂ gas is acidic in nature.

Question 13.
Starting from carbon dioxide how would you obtain

(a) A weak acid
(b) A fertilizer
(c) A highly poisonous gas
(d) Black particles of carbon.

[Give balanced equations for the same]
Answer:

(a) Carbonic acid from CO₂ : when CO₂ is dissolved in water, carbonic acid is formed.

(b) CO₂ reacts with ammonia gas to form urea (a nitrogenous fertilizers).

(c) CO₂ is reduced to a highly poisonous gas CO by carbon (coke).

(d) Black particles of carbon : Burning magnesium burns in CO₂ making use of oxygen of CO₂ producing black carbon particles.

Question 14.
State how you would convert carbon dioxide to a metallic carbonate using a basic oxide e.g. sodium oxide. [Give a balanced equation]
Answer:
Basic oxide like Na₂O by CO₂ are converted into carbonate.
Na₂O + CO₂ → Na₂CO₃

Question 15.
‘When carbon dioxide is bubbled into lime water, the lime water turns milky and when bubbled in excess the milkiness disappears’. Give balanced equations to support the statement.
Answer:

When CO₂ is passed in excess, milkiness disappears due to formation of bicarbonate which is soluble in water.

Question 16.
Explain the term ‘dry ice’. State its application. Give three reasons why carbon dioxide finds application in fire extinguishers.
Answer:

• Dry ice : Solid carbon dioxide is called dry ice.
• Applications : Dry ice is used as refrigerant for preservation of foods as it freezes faster and lasts longer than ordinary ice.

Question 17.
Using a magnesium ribbon, how would you prove that a given gas jar contains carbon dioxide.
Answer:
It is a test to find gas in the jar is carbon dioxide. On burning magnesium ribbon in a jar of carbon dioxide.

Mg bums in CO₂ gas depositing – black particles of carbon on the walls of the jar. If black particles are found on the walls of jar, it is carbon dioxide gas.
_CO2 + 2Mg → 2MgO + C

Question 18.
State the function of cone, sulphuric acid in the laboratory preparation of carbon monoxide from oxalic acid.
Answer:
Function of cone, sulphuric acid is to remove molecules of water from formic acid i.e. it acts as dehydrating agent.

Question 19.
Give reasons why carbon monoxide is considered a highly poisonous gas. State why it is dangerous

1. To sleep in a closed room with a fuel burning
2. To be in the vicinity of smokers.

Answer:
Carbon monoxide is highly Poisonous gas : Carbon-monoxide combines with haemoglobin oxygen cariying capacity of blood is decreased

1. In a room which is closed and coal fire produces CO in limited supply of oxygen as room is closed. The haemoglobin combines with CO to form carboxy- -haemoglobin and oxygen carrying capacity decreases which causes death.
2. To be in vacinity of smokers : The fumes of – burning tobacco contains traces of carbon monoxide.

Question 20.
Convert carbon monoxide to carbon dioxide using two different methods.
Answer:
Methods of converting CO to CO₂.

1. CO bums in oxygen and changes to C02.
   
2. By reducing metal oxide.
   ZnO + CO → Zn + CO₂

Question 21.
State how carbon monoxide finds application in

1. The metallurgy of iron
2. Preparation of an alcohol.

[Give balanced equations for the same]
Answer:

1. CO is used to extract iron metal from its oxide as it reduces heated metal oxide.
   
2. Preparation of alcohol :
   
   

OBJECTIVE TYPE QUESTIONS

Q.1. Name the following :

Question 1.
A crystalline allotrope of carbon built up from a hexagonal unit.
Answer:
Graphite.

Question 2.
An allotrope of carbon used for the manufacture of coke.
Answer:
Coal.

Question 3.
An amorphous allotrope of carbon which floats on water.
Answer:
Wood charcoal.

Question 4.
An acid formed when carbon dioxide is dissolved in water under pressure.
Answer:
Carbonic acid.

Question 5.
The gaseous product obtained when carbon monoxide is passed over heated iron (III] oxide.
Answer:
CO₂ [Carbon dioxide].

Q.2. Select the correct answer from A, B, C, D & E for each statement given below :
A : Anthracite
B : Diamond
C : Carbogen
D : Urea
E: Lampblack

Question 1.
An allotrope of carbon used as a tip for deep boring drills.
Answer:
B : Diamond

Question 2.
The type of coal with the highest carbon conent.
Answer:
A : Anthracite

Question 3.
An allotrope of carbon obtained by burning kerosene oil in a limited supply of air.
Answer:
E : Lampblack

Question 4.
A nitrogenous fertilizer obtained from carbon dioxide.
Answer:
D : Urea

Question 5.
A compound which finds use for a victim of carbon monoxide poisoning.
Answer:
C : Carbogen

Q.3. Give a balanced equation for the following conversions : [In one or two steps]

Question 1.
Coke to water gas.
Answer:
Steam is passed over white hot coke at 1000°C

Question 2.
Calcium bicarbonate to calcium nitrate using a dilute acid.
Answer:
Calcium bicarbonate to calcium nitrate

Question 3.
Lime water [soln. of calcium hydroxide] to calcium bicarbonate.
Answer:
Lime water to calcium bicarbonate
When CO₂ is passed it turns milky.

Question 4.
Carbon dioxide to carbon.
Answer:
CO₂ to carbon
On burning magnesium in CO₂

Question 5.
A metallic oxide to calcium carbonate.
Answer:
A metallic oxide to calcium carbonate

Q.4. Complete the statements by filling the blanks with the correct word from the bracket.

Question 1.
The crystal of ___ [diamond/graphite] is opaque to light and is a good conductor of heat.
Answer:
The crystal of graphite is opaque to light and is a good conductor of heat.

Question 2.
A graphite-clay mixture baked at high temperature is used in making ___ [lubricants/refractory cruscibles].
Answer:
A graphite-clay mixture baked at high temperature is used in making refractory cruscibles.

Question 3.
Adsorption capacity of wood charcoal is increased by passing ___ [carbon dioxide/steam] over wood charcoal, at high temperature.
Answer:
Adsorption capacity of wood charcoal is increased by passing steam over wood charcoal, at high temperature.

Question 4.
___ [organic/inorganic] Chemistry is the Chemistry of carbon compounds mainly of ‘Carbon’, ‘Hydrogen’ & ‘Oxygen’.
Answer:
Organic Chemistry is the Chemistry of carbon compounds mainly of ‘Carbon’, ‘Hydrogen’ & ‘Oxygen’.

Question 5.
Sodium oxide combines with carbon dioxide to give ___ (sodium carbonate/sodium bicarbonate].
Answer:
Sodium oxide combines with carbon dioxide to give sodium carbonate.

Q.5. Give reasons for the following :

Question 1.
Diamond & graphite are allotropic modifications of carbon.
Answer:
Both diamond and graphite are carbon element, differ only in physical state and properties. Both on burning form CO₂ and nothing is left.

Question 2.
It is dangerous to stand behind a running engine of a vehicle.
Answer:
Running engine of a vehicle produces carbon monoxide which is dangerous for us.

Question 3.
Both CO₂ & SO₂ turn lime water milky. Moist potassium permanganate paper, helps in distinguishing the two gases.
Answer:
SO₂ gas moist potassium permanganate paper turns purple to colourless when comes in contact with sulphur dioxide gas where as CO₂ gas has no effect.

Question 4.
Carbon monoxide and not carbon dioxide is a highly poisonous gas.
Answer:
CO is highly poisonous combines with haemoglobin of R.B.C. and reduces the oxygen carrying capacity and death results.

Question 5.
Lime water finds application for testing both carbon dioxide & carbon monoxide gas individually.
Answer:
CO carbon-monoxide when passed through lime water, does not produce milkiness where as CO₂ produces milkiness in lime water and hence is used to test CO and CO₂.

New Simplified Chemistry Class 8 ICSE Solutions – Water

ICSE SolutionsSelina ICSE SolutionsML Aggarwal Solutions

Simplified ChemistryChemistryPhysicsBiologyMathsGeographyHistory & Civics

EXERCISE

Question 1.
Name a ‘pure form’ and an,‘impure form’ of water in the natural state.
Answer:
Rain water – pure natural water.
Sea water, lake water – impure form of natural water.

Question 2.
State what is potable water. Give its qualities or characteristics which make it fit for human consumption.
Answer:
Potable water : “That water which is fit for human consumption and for drinking purposes.”
Characteristics of potable water :

1. Water should be free from harmful bacteria, germs, suspended impurities and harmful salts.
2. Should be clear, colourless and odourless.
3. Should contain small amounts of dissolved gases and minerals for good taste.
4. Should have dissolved air and salts of (Na, Ca, Mg) which impart taste to the water and are useful for metabolic functions of the human body.

Question 3.
Give a reason why water is called a ‘universal solvent’ but, an alkali is not.
Answer:
Water is a polar covalent compound. When it comes in contact with any substance it breaks the electrostatic forces holding the molecules of that substance. Thus, the molecules break loose from the substance and hence dissolve in water. Thus, water is called a universal solvent and an alkali is not.

Question 4.
Name the solute, solvent & solution in the statement – ‘sodium chloride dissolves in water to give sodium chloride solution’. Define each of the terms in italics.
Answer:

• Solute (sodium chloride) : The substance which dissolves or disappears in solvent, i.e. liquid to form a solution.
  Mostly solvent is which is in less quantity.
• Solvent (water) : The substance which allows the solute to dissolve in it. It is the substance which is in more quantity.
• Solution (sodium chloride solution) : The homogenous mixture of solute and solvent is called solution.

Question 5.
Explain how each of the following factors affects the formation of a solution

(a) Proportion of the solute & the solvent
(b) Particle size of the solute
(c) Temperature of the solvent

Answer:
Factors effecting the formation of solution :

(a) Proportion of solute & the solvent : The solute must be in a small amount compared to the solvent which must be in much greater amount to dissolve the solute.
(b) Particle size of the solute : Smaller the size of solute, quickly and completely they will dissolve and form solution.
(c) Temperature of the solvent : Sometime increase in temperature makes the solution quicker.

Question 6.
Define the term ‘solubility ’ of a solute in water. If 10 g. of a solute is added to 100 g. of water at 50°C and stirred, it is seen that the total solute dissolves at that temperature giving an unsaturated solution. Is 10 g, the solubility of the solute in water. Give reasons.
Answer:
Solubility is the maximum amount of solute in grams – that will saturate 100 g of water at 0°C.
OR
Maximum amount of solute that can be dissolved in 100 g of solvent at specific temperature.
No, 10 g is not solubility as the solution is unsaturated and not saturated solution. This means more of solute can be added to make it saturated solution.

Question 7.
Give a reason why – (a) Alcohol and water form a miscible mixture while oil & water do not. (b) Boiled water tastes flat, (c) On opening a bottle of soda [carbon dioxide dissolved in water] the gas escapes out with a ‘fizz’.
Answer:

(a) Alcohol and water form miscible mixture as alcohol is soluble in water. Oil does not dissolve in water form immiscible mixture.
(b) Boiled water loses the gases and mineral and the water tastes flat.
(c) Solubility of gases in liquid decreases on decreasing the pressure and CO₂ come out of water with a ‘fizz’ when pressure decreases on opening the bottle.

Question 8.
Give the importance of

(a) Dissolved minerals & salts
(b) Dissolved air – in water.

Answer:
Importance in water of:

(a) Dissolved minerals & salts : Provide taste to water and are useful for metabolic functions of the human body.
(b) Air : Air containing various gases gives taste to water.

Question 9.
How are solutions generally classified. What is the basis of this classification.
Answer:
Solutions are classified into :

(a) True solutions.
(b) Colloidal solution.
(c) Suspensions.

The basis of classification is the size of dissolved or dispersed particles in the medium.

Question 10.
Draw a table to differentiate – True solutions, colloidal solutions & suspensions – with respect to

1. Type of mixture
2. Solute particles in the mixture.

Answer:
True solution :

1. Homogeneous mixture.
2. Particles are extremely small less than 1 nm in diameter = 10^-9 m.

Colloidal :

1. Appears to be homogeneous actually it is heterogeneous
2. Between 1 nm and 100 nm in diameter.

Suspension :

1. Heterogeneous mixture.
2. Size larger than 100 nm in diameter. Quite large size.

Question 11.
Give four examples of each of the above types of solutions i.e. true solution, colloidal solution & suspension. Give a reason why a true solution does not exhibit ‘Tyndall effect’.
Answer:
Examples of :

(a) True solution : Salt solution, vinegar, copper sulphate solution, sugar in water solution, air, brass.
(b) Colloidal solution : Milk, ink, blood, soap solution, starch sol.
(c) Suspension : Milk of magnesia, flour in water, chalk- water solution, muddy water. A true solution cannot exhibit ‘Tyndall effect’ because ‘the size of particles is very small and cannot reflect light falling on them.”

Question 12.
If 5 g. of a solute is added to 50 ml. of a solvent at a particular temperature and the solution can dissolve more of the solute at that temperature – is the solution obtained a saturated or an unsaturated solution. Give reasons.
Answer:
As more of solute can be dissolved at that temperature, it is unsaturated solution because saturated solution cannot dissolve any more quantity of solute at a particular temperature.

Question 13.
Explain the meaning of the term ‘saturated solution’. State two methods to convert a saturated solution to an unsaturated solution.
Answer:
Saturated solution : “A solution in which no more solute can be dissolved at that temperature, is called saturated solution.”
Two methods to convert a saturated solution into unsaturated solution are :

1. By increasing temperature
2. By increasing the amount of solvent.

Question 14.
State how a ‘supersaturated’ solution differs from a ‘saturated solution’. State briefly how you would prepare a supersaturated solution using potassium chloride and water.
Answer:
Super saturated solution contains more amount of solute than saturated solution in hot solution. On cooling, it gives out excess salt which separate out.
Preparation of supersaturated solution of potassium chloride in water : Take 100 g of water and add little by little of KCl salt and stir it, if it gets dissolved, add a little more and stir it. We find that 35 g of KCl can make the solution saturated at 20°C. Now heat the saturated solution and add more of KCl. At 40°C it will dissolve 40 g and 46 g at 60°C and 54 g at 80°C. This super saturated solution at 80°C. On cooling this super saturated solution excess of solute [KCl] crystals separates out.

Question 15.
The process of separation or deposition of crystals from a hot saturated solution on gentle cooling of the solution is called ‘crystallisation’. With reference to the statement explain what is meant by ‘water of crystallisation’. State which physical properties of a crystal are correlated with the water of crystallisation.
Answer:
Water of crystallisation is the number of water molecules that combine chemically – in definite proportion with the – concerned salt in the crystalline state.
OR
Some salts, while crystallising out from their solutions unite with a definite quantity of water, which is known as water of crystallisation.
Physical properties are that crystals are solid, definite regular shapes.

Question 16.
Differentiate between ‘hydrated’ & ‘anhydrous’ crystals. Give the chemical formula of – the following hydrated crystals –

(a) Washing soda
(b) Gypsum
(c) Blue vitriol
(d) Epsom salt
(e) Glauber’s salt. State

which crystal from (a) to (e) is a pentahydrate.
Answer:
Hydrated crystals : Contain definite number of water molecules in loose chemical combination with the crystals.
Anhydrous crystals : Do not contain any definite number of water molecules – in loose chemical combination with the crystals.

Formula of hydrated crystals :

(a) Washing soda : CaCO₃10H₂O
(b) Gypsum : CaSO₄.2H₂0
(c) Blue vitriol : CuSO₄.5H₂0
(d) Epsom salt : MgSO₄.7H₂0
(c) blue vitriol [CuSO₄.5H₂0] is a pentahydrate.

Question 17.
Differentiate between an efflorescent, deliquescent & ‘ hygroscopic substance with suitable examples.
Answer:
Efflorescent substances :

1. Lose wholly or partially their water of crystallisation when exposed to air.
2. Become powdery.
3. Washing soda CaCO₃.10H₂O, Gauber’s salt
   
4. Crystalline when hydrated.

Deliquescent substances :

1. Absorb moisture from atmosphere, absorb water and dissolve in it forming a saturated solution.
2. Deliquescence is maximized in dry conditions,
3. NaOH, KOH, CaCl₂, MgCl₂, FeCl₃ are the examples.
4. They are crystalline water soluble.
   FeCl₃ → FeCl₃ [Crystal changes to Solution]

Hygroscopic substances :

1. They are amorphous solids or liquids.
2. Absorb moisture from the air of atmosphere.
3. Do not change their original state.
   CaO (quick lime), cone. H₂SO₄, P₂O₅ and silica gel are the examples.
   
   (State not changed)
   is used for drying gases in laboratory.

Question 18.
Differentiate the function of concentrated sulphuric acid as a drying agent & as a dehydrating agent. Is fused calcium chloride a dehydrating or a desiccating agent. Give reasons.
Answer:
Sulphuric acid (cone.) acts as drying agent when it removes water from other substances i.e. for drying gases.
Cone, sulphuric acid acts as dehydrating agent when it removes chemically combined water from compounds due to it strong affinity for water.

Cone. H₂SO₄ has removed H₂0 from ethanol and changed it to ethene. Fused calcium chloride [CaCl₂] is desiccating agent as it is drying the substance.

Question 19.
Give a reason why metals are arranged in a series called – ‘activity series of metals’. Name a metal which reacts
with

(a) Cold water
(b) Boiling water
(c) Steam to liberate hydrogen in each case.

Answer:
Metals arranged in series is called ‘activity series of metals’ because metals differ in tendency to lose valence electrons and can be arranged in a series, i.e. metals are arranged in decreasing order of their reactivity. The most reactive metal at top of the series and then with decreasing reactivity.
Metal that react with

(a) Cold water – Sodium

(b) Boiling water – Magnesium [Mg]
(c) Steam – Iron [Fe]

Question 20.
Differentiate between – (a) Hard & soft water (b) Temporary & permanent hard water – with suitable examples.
Answer:
(a) Hard water : It does not lather readily with soap and soap is wasted.
Soft water : Soft water form lather easily with soap.

(b) Temporary hard water : contains bicarbonate of calcium or magnesium. Hardness can be removed by boiling water.
Permanent hard water : contains sulphates or chlorides of calcium and magnesium. Hardness cannot be removed by boiling water.

Question 21.
Give balanced equations to show how – (a) Temporary hardness enters into water, (b) Temporary hardness in water can be removed by boiling, (c) Permanent hardness in water can be removed by addition of washing soda.
Answer:

(a) Temporary hardness : Enters in water when rain water containing CO₂ falls on earth and combines with limestone [CaCO₃] & magnesite [MgCO₃] layers forming respective soluble bicarbonates.

(b) To remove temporary hardness by boiling : Bicarbonate changes to insoluble carbonate and water becomes soft.

(c) Removal of permanent hardness in water by adding washing soda : CaCl₂ or CaSO₄ present in permanent hard water exchanges the ions and forms CaCO₃ which being in soluble can be filtered.

OBJECTIVE TYPE QUESTIONS

Q.1. Select the correct answer from A, B, C, D & E for each statement given below :
A : Colloidal
B : Fused calcium chloride
C : Solvent
D : Suspension
E : Washing soda

Question 1.
The medium of dissolution which allows the solute to dissolve in it.
Answer:
C : Solvent

Question 2.
A solution which can pass through a filter paper but not through a semipermeable membrane.
Answer:
A : Colloidal

Question 3.
A decahydrate crystal.
Answer:
E : Washing soda

Question 4.
A drying agent placed in desiccator.
Answer:
B : Fused calcium chloride

Question 5.
A heterogenous mixture of undissolved particles in dispersion medium, visible to the naked eye.
Answer:
D : Suspension

Q.2. Give a balanced equation for the following conversions :

Question 1.
Calcium sulphate in permanent hard water to calcium carbonate using sodium carbonate.
Answer:

Question 2.
Iron to triiron tetroxide using steam.
Answer:

Question 3.
Sulphur dioxide to sulphurous acid using a neutral liquid.
Answer:

Question 4.
Potassium oxide to a strong alkali.
Answer:

Question 5.
Magnesium bicarbonate in temporary hard water to magnesium carbonate by boiling.
Answer:

Q.3. Complete the statements by filling the blanks with the correct word from the bracket.

Question 1.
Solubility of most solids ___ [decreases/increases] with rise in temperature.
Answer:
Solubility of most solids increases with rise in temperature.

Question 2.
Kerosene & water form a ___ [miscible / immiscible] mixture.
Answer:
Kerosene & water form a immiscible mixture.

Question 3.
Solubility of a solute is the ___ [minimum/maximum] amount of solute that will saturate 100 g. of water at t°C.
Answer:
Solubility of a solute is the maximum amount of solute that will saturate 100 g. of water at t°C.

Question 4.
Hygroscopic substance absorb moisture from the atmosphere & ___ [do not change/change] their original state.
Answer:
Hygroscopic substance absorb moisture from the atmosphere & do not change their original state.

Question 5.
The ratio of hydrogen & oxygen in water is ___ [2 : 1/ 1 : 2].
Answer:
The ratio of hydrogen & oxygen in water is 2 : 1.

Q.4. Give reasons for the following :

Question 1.
All solutions are homogenous mixtures of a solute in a solvent.
Answer:
Because solute dissapears in solvent and uniform composition throughout.

Question 2.
Hardness in temporary water can be removed by boiling, but hardness in permanent hard water cannot.
Answer:
Temporary hardness is due to the presence of bicarbonate of calcium or magnesium, on boiling bicarbonate decomposes to CO₂, carbonate which is in soluble and can be filtered out. Where as permanent hard water contains [SO₂]^2- or Cl^–– of calcium and magnesium, cannot be removed by boiling.

Question 3.
Colloidal solutions exhibit brownian movement.
Answer:
Particles in colloidal solutions do not settle down and are always in motion.

Question 4.
The percentage of oxygen, in air dissolved in water, is higher than the percentage of oxygen in ordinary air.
Answer:
Oxygen dissolves in water and in air there are some other gases also which make the bulk of air. Some gases dissolve in water.

Question 5.
Washing soda can be used to remove both temporary and permanent hardness in water.
Answer:
Washing soda Na₂CO₃ exchanges ions [SO₄]^– and Cl^– and do not make water hard.

Q.5. Match the substances in List I with the appropriate answer in List II.

Answer:

New Simplified Chemistry Class 8 ICSE Solutions – Hydrogen

ICSE SolutionsSelina ICSE SolutionsML Aggarwal Solutions

Simplified ChemistryChemistryPhysicsBiologyMathsGeographyHistory & Civics

EXERCISE

Question 1.
State how hydrogen occurs in the free state. Name three compounds containing hydrogen in the combined state.
Answer:

• In free state : In traces in the earth’s crust and atmosphere and in volcanic gases. It is also found in huge amount in the interior of the sun.
• In combined state : In organic compounds, plant and animals, in acids, in water.

Question 2.
Starting from zinc how would you obtain hydrogen using

(a) Steam
(b) A dilute acid
(c) An alkali

[ Give balanced equations for each & name the product formed in each case other than hydrogen]. Name a metal which will not react with the reactants above to give hydrogen.
Answer:
To obtain hydrogen using :

(a) Steam : Metals above [H] in activity series of metals, i.e. Zn, Fe and Pb react with steam and form corresponding oxides.

Metals below [H] do not react even with steam. Metal are Cu, Hg, Ag, Pt and Au.
(b) With dil. acids : Metals like Zn, Fe, Pb form corresponding salt.

Cu and metals below Cu do not react with dil. acids.
(c) An alkali : Zn, Pb react with concentrated alkali on boiling to give the corresponding metallic salt and liberate hydrogen.

Question 3.
‘Hydrogen is obtained by electrolysis of acidified water’. Answer the following pertaining to the preparation of hydrogen by electrolysis, (a) The meaning of the term ‘electrolysis’ and ‘electrolyte’, (b) Name the electrode –

1. through which the current enters the electrolyte.
2. at which hydrogen is liberated.

Answer:

(a) Electrolysis : “Dissociation of water into hydrogen gas and oxygen gas by the passage of electric current is called electrolysis.”

• Electrolyte : “A chemical compound (water) in solution state which conducts electric current and decomposes is called electrolyte.”

(b)

1. Electrode through which current enters the electrolyte is anode.
2. Electrode at which hydrogen is liberated is cathode.

Question 4.
In the laboratory preparation of hydrogen from zinc & dilute hydrocholoric acid – state a reason for

(a) Addition of traces of copper [II] sulphate to the reaction medium
(b) Collecting the hydrogen by downward displacement of water and not air & collecting it after all the air in the apparatus is allowed to escape
(c) Having the end of the thistle funnel dip below the level of the acid in the flask.

Answer:

(a) Addition of traces of copper [II] sulphate acts as catalyst and speeds up the reaction.
(b) Hydrogen is collected by downward displacement of water as it is slightly soluble in water and in air it form explosive mixture with air and also hydrogen is lighter than air.
(c) Otherwise the gas formed will escape through thistle funnel.

Question 5.
In the industrial method of preparation of hydrogen by the Bosch process – give

(a) Balanced equations for the first two main steps in the production of hydrogen
(b) The reason for use of addition of a promoter to the catalyst in the final step
(c) The name of the solution which absorbs the unreacted carbon monoxide.

Answer:
(a) Bosch process for industrial production of hydrogen :

• Step [I] : Preparation of water gas [CO + H₂] by passing steam over white hot coke.
  
• Step [II] : Reduction of steam to hydrogen by carbon monoxide.
  

(b) Promotor Cr₂O₃ is used to increase the efficiency of catalyst [Fe₂O₃].
(c) Ammoniacal cuprous Chloride [CuCl] solution absorbs unreacted CO.

Question 6.
State the following pertaining to the physical properties of hydrogen :

(a) Colour & odour
(b) Solubility in water
(c) Effect on moist blue litmus paper.

Answer:

(a) Colour and odour of hydrogen gas colourless. Odourless.
(b) Slightly soluble in water.
(c) No-effect i.e. colour does not change.

Question 7.
Draw neat labelled diagrams for two different experiments to prove that – hydrogen is lighter than air.
Answer:

1. To show that H₂ is lighter than air :
   
   
2. Air from jar B being heavier runs down in jar A and H₂ runs to jar B as it is lighter than air and burns with ‘pop’ sound in jar B if a burning splinter is brought there.

Question 8.
Starting from hydrogen gas how would you obtain

(a) A neutral liquid
(b) A basic gas
(c) A metal by reduction of its heated oxide.

[The metal formed is above iron in the activity series]
Answer:

(a) Hydrogen bums quietly in oxygen to produce water (a neutral liquid)
2H₂ + O₂ → 2H₂0
(b) 3 volumes of H₂ react with 1 vol. of N₂ gas in presence of catalyst (finely divided Fe at 450-500°C) and a basic gas ammonia is produced.

(c) Reduction of Fe₂O₃ (iron oxide) on heating to metal iron.

Question 9.
Using a burning candle and a jar of hydrogen – how would you prove experimentally that (a) Hydrogen is a combustible gas (b) Hydrogen does not support combustion.
Answer:

(a) Hydrogen gas is combustible and bums at the mouth of the gas jar. When a burning candle is brought near the mouth of hydrogen in gas jar.

(b) Hydrogen extinguishes the burning candle when pushed inside the jar. This shows that hydrogen does not support combustion.

Question 10.
State a reason why, when hydrogen is passed over heated copper oxide, the resultant product formed, differs in colour from the original reactant.
Answer:

H₂ removes oxygen from CuO and reduces it to Cu metal (reddish brown in colour).

Question 11.
With reference to the uses of hydrogen, give reasons for the following :

(a) Hydrogen is not used in air balloons
(b) A mixture of hydrogen & oxygen on burning, find application in welding & cutting metals
(c) Reaction of hydrogen with nitrogen under specific conditions finds industrial utility.

Answer:

(a) Hydrogen gas in ‘highly inflamable’ i.e. catches fire. Hence generally it is not used in air balloons.
(b) Hydrogen and oxygen mixture on burning produces a high temperature upto 2800°C which is used for welding and cutting of metals.
(c) One vol. of N₂ and 3 vol. of H₂ in presence of a catalyst (finely divided iron) at 450-500°C and at 200-900 mm pressure forms basic compound ammonia [NH₃] which finds applications in fertilizers and nitric acid [HNO₃] and explosives.

Question 12.
Give a test to differentiate between two gas jars – one containing pure hydrogen and the other hydrogen-air mixture.
Answer:
When a burning splinter is brought near the gas jar containing, pure H2 bums with pale blue flame quietly.
H₂ + Oxygen mixture bums with explosion.

Question 13.
With reference to oxidation & reduction reaction – complete the statement given by filling in the blanks with only the words (a) Addition (b) Removal.
‘Oxidation is a chemical reaction involving ____ of oxygen to a substance or ____ of hydrogen from a substance. Reduction on the otherhand involves ____ of hydrogen to a substance or ____ of oxygen from a substance.
Answer:
‘Oxidation is a chemical reaction involving – addition of oxygen to a substance or removal of hydrogen from a substance. Reduction on the otherhand involves addition of hydrogen to a substance or removal of oxygen from a substance.

Question 14.
With reference to the equation : Cl₂ + H₂S → 2HCl + S pertaining to a redox reaction – select the correct answer in each case –

(a) Chlorine is oxidised/redcued to HCl.
(b) Hydrogen sulphide is oxidised/redcued to sulphur since the reaction involves addition/removal of hydrogen.
(c) Chlorine acts as an oxidising/reducing agent.

Answer:

OBJECTIVE TYPE QUESTIONS

Q.1. Give balanced equations for the following conversions :

Question 1.
Zinc to sodium zincate – using an alkali.
Answer:
Zinc to sodium zincate.

Question 2.
Acidified water to hydrogen – by electrolysis.
Answer:

Question 3.
Water gas to hydrogen – industrially.
Answer:

CO₂ is removed by dissolving the mixture obtained in water under pressure
By passing the mixture through water or KOH [Caustic Potash Solution]
2KOH + CO₂ → K₂CO₃ + H₂0
Residual gas left is HYDROGEN.

Question 4.
Iron [III] oxide to iron – using hydrogen.
Answer:
Fe₂O₃ is reduced by hydrogen

Question 5.
Nitrogen to a basic gas – using hydrogen.
Answer:
Nitrogen to basic gas [NH₃] – using H₂

Q.2. Give reasons for the following :

Question 1.
Copper does not displace hydrogen from dilute hydrochloric acid, but zinc does.
Answer:
Copper lies below [H] in activity series of metals and cannot displace hydrogen from acid.
Zn lies above [H] in reactivity series of metals and being more reactive can displace hydrogen from dil. acids.

Question 2.
In the preparation of hydrogen by electrolysis of water – the distilled water used is acidified.
Answer:
Pure or distilled water is bad conductor of electricity and does not form ions. To increase the number of H⁺ ions distilled water is acidified and made good conductor of electricity.

Question 3.
In the laboratory preparation of hydrogen from zinc and dilute hydrochloric acid – the zinc used granulated zinc.
Answer:
Granulated zinc contains traces of impurities which act as catalyst and increase the rate of production of hydrogen.

Question 4.
In Bosch process – the final gaseous products are passed through caustic potash [KOH] soln.
Answer:
In Bosch process, final products are passed through caustic potash solution to separate hydrogen from the mixture. CO₂ dissolves in KOH leaving behind hydrogen.

Question 5.
The reaction of chlorine with hydrogen sulphide is deemed a redox reaction.
Answer:
Reaction of Cl₂ with H₂S is redox reaction as Cl₂ gets reduced to HCl and H₂S is oxidised simultaneously in the reaction. Hence it is redox reaction.

Q.3. Select the correct answer from A, B, C, D or E for each statement given below :
A: Nickel
B: Sodium
C: Iron
D: Iron [III] oxide
E: Magnesium oxide

Question 1.
A metal which reacts with water to give a metallic hydroxide & liberate hydrogen.
Answer:
B : Sodium

Question 2.
The metallic compound used as a catalyst in Bosch process.
Answer:
D : Iron [III] oxide

Question 3.
The metal used as a catalyst in hydrogenation of oils.
Answer:
A : Nickel

Question 4.
The metal which reacts with steam liberating hydrogen & the reaction is reversible.
Answer:
C : Iron

Question 5.
The metallic compound formed when a metal above aluminium in the activity series reacts with steam.
Answer:
E : Magnesium oxide

Q.4. Select the correct answer from the choice in bracket to complete each sentence.

Question 1.
The acid ___ [dil.H₂SO₄, dil. HNO₃, dil. HCl] is not Used in the laboratory preparation of hydrogen, using zinc and an acid.
Answer:
The acid dil. HNO₃ is not used in the laboratory preparation of hydrogen, using zinc and an acid.

Question 2.
In Bosch process the catalytic reduction of steam to hydrogen is carried out by ___ [CO₂, CO, C].
Answer:
In Bosch process the catalytic reduction of steam to hydrogen is carried out by CO [carbon monoxide].

Question 3.
A foul smelling gas formed when hydrogen reacts with a molten non-metal, is ___ [hydrogen chloride, hydrogen sulphide, ammonia].
Answer:
A foul smelling gas formed when hydrogen reacts with a molten non-metal, is hydrogen sulphide [H₂S].

Question 4.
The product formed on combustion of hydrogen in air is ___ [water gas, water, producer gas].
Answer:
The product formed on combustion of hydrogen in air is water.

Question 5.
The gas which has now replaced hydrogen in air balloons is ___ [argon, helium, neon].
Answer:
The gas which has now replaced hydrogen in air balloons is helium.

Q.5. Match the statements in List I with the appropriate answer in List II.

Answer:

New Simplified Chemistry Class 8 ICSE Solutions – Chemical Reactions

ICSE SolutionsSelina ICSE SolutionsML Aggarwal Solutions

Simplified ChemistryChemistryPhysicsBiologyMathsGeographyHistory & Civics

EXERCISE

Question 1.
State what is a chemical reaction. A chemical reaction is often accompanied by external indications or characteristics which include – (a) Colour change (b) Effervescene or gas evolved (c) Evolution or absorption of heat (d) Formation of a precipitate. With reference to each of the above indications, state the external indication seen during – (i) Addition of dilute acid to an active metal (ii) Addition of dilute hydrochloric acid to silver nitrate (iii) Addition of water to quicklime (iv) Thermal decomposition of mercury [II] oxide.
Answer:
Chemical reaction : “Is a chemical change in which matter changes into a new substance or substances.”

1. Addition of dil. acid to an active metal : Gas is evolved, test tube or flask becomes hot (heat is produced).
2. Addition of dil. hydrochloric acid to silver nitrate : A white coloured precipitate of AgCl is seen.
3. Addition of water to quick lime : Hissing sound accompanied by a lot of heat (boiling).
4. Thermal decomposition of mercury [II] oxides : Red colour of Hg[II]0 changes to silvery (Hg metal).

Question 2.
State why a direct combination reaction is called a – ‘synthesis reaction’.
Answer:
When two or more substances [element with element or element with compound or compound with another compound] combine to form a new compound with new properties. Hence direct combination is called ‘synthesis reaction’.
Example : Hydrogen (a gas) + Oxygen (a gas) combines to form water (liquid).
2H₂ + O₂ → 2H₂0 H₂

Question 3.
Differentiate between –

(a) Direct combination reaction & a decomposition reaction
(b) Displacement reaction & a neutralization reaction.

Answer:
(a) Direct combination : “Is a reaction in which two elements or compound combine to give one new compound.”
C + O₂ → CO₂

• Decomposition reaction : “A chemical compound (breaks up) decomposes into two or more simpler substances.” Copper carbonate → Copper oxide + carbon dioxide
  CUCO₃ → CuO + CO₂
  

(b) Displacement reaction : “A chemical reaction in which an element displaces the element from a solution of its compound.”

In a activitiy series of metals, an element placed higher displaces the element placed below it.

Here Zn is placed above Cu in activity series of metal and is more reactive than copper has displaced Cu from copper sulphate.

• Neutralization reaction : “A reaction between an acid and a base to form salt and water is called neutralization reaction.”
  

Question 4.
Classify the following reactions into – (a) Direct combination (b) Decomposition (c) Displacement (d) Double decomposition – The reactions are – (i) Zinc hydroxide on heating gives zinc oxide & water (ii) Zinc reacts with copper [II] sulphate to give zinc sulphate & copper (iii) Zinc sulphate reacts with ammonium hydroxide to give ammonium sulphate & zinc hydroxide (iv) Molten zinc at high temperatures, burns in air to give zinc oxide.
Answer:

1. Zinc hydroxide a single compound decomposes into two simpler components zinc oxide and water, hence it is decomposition reaction.
2. Zinc reacts with copper [II] sulphate to give zinc sulphate and copper is displacement reaction. Zinc being more reactive than copper displaced copper from copper sulphate.
3. Is double decomposition as exchange of ions takes place.
   

Question 5.
Give balanced equations for –

1. A direct combination reaction involving two elements, one of which is a non-metal
2. A thermal decomposition reaction involving heat on limestone [calcium carbonate]
3. An electrolytic decomposition reaction involving a neutral liquid
4. A displacement reaction involving a metal above hydrogen in the activity series with copper [II] sulphate solution
5. A double decomposition neutralization reaction involving an acid & a base
6. A white precipitate obtained during a double decomposition reaction involving a silver salt with a sodium salt.

Answer:

1. Balanced chemical equation of : A combination reaction between calcium and oxygen (non-metal) to form calcium oxide.
   2Ca + O₂ → 2CaO
2. Thermal decomposition of limestone :
   
   
3. Electrolytic decomposition of neutral liquid [water]
   
4. Displacement reaction
   
   [more reactive displaces less reactive Cu]
5. Neutralization of acid with base to form salt and water.
   
6. Double decomposition reaction
   

Question 6.
State what is meant by ‘reactivity series of metals’. With reference to – (a) Water (b) Acids explain with suitable examples how the reactivity of the metals could be differentiated.
Answer:
Activity series of metals : “Is a series of arrangement of metals in decreasing order of their reactivity.”
i.e. metals at top are most reactive and least reactive metals are at the bottom of the series. Metal above is more reactive than the lower metal. In other words K is more reactive than all the metals below it and Na is more reactive than all the metals below it.
(a) Action of metals with water : K, Na, Ca react with cold water.
K : Darts on water surface and react violently.
Na : Revolves on the water surface and bums.
Ca : Sinks in water, react less violently.
Hence, K > Na > Ca calcium is less reactive than K and Na.
Mg and Al react with boiling water/steam.
Hence, Mg and Al are less reactive than K, Na and Ca. Zn, Fe, Pb also react with steam and reaction stops soon. Fe when hot reacts with steam
This shows that Mg > Al > Zn > Fe > Pb.
Cu, Hg, Ag, Pt, Au do not react with steam or even when hot are less reactive.
(b) Action of metals with acids : As reactivity decreases from top to bottom in metal activity series.
K and Na react with dil. HC1 and dil. H₂SO₄ explosively to produce H₂↑
2Na + 2HCl → 2NaCl + H₂
Ca, Mg, Al, Zn and iron react less vigorously with decreasing vigour with dil. H₂SO₄ or dil. HC1 to produce H₂↑
Ca + H₂SO → CaSO₄ + H₂
Metals below hydrogen do not react with dil. acid and do not displace H₂ from it.

Question 7.
A chemical reaction may be ‘reversible’ in nature. State the meaning of the term in italics. Give a reason why a catalyst is used in certain chemical reactions. Give a r balanced equation for the following – (a) A reversible catalytic reaction involving –

1. nitrogen as one of the reactants
2. sulphur dioxide as one of the reactants.

Answer:
Reversible reaction : “A chemical reaction is said to be reversible in nature if “products formed react together – to form actual reactants depending on the condition of the reaction.”

A catalyst is used to make the reaction fast or slow down it. When KClO₃ is heated, oxygen is produced but reaction is very slow. If we add MnO₂ to KClO₃ and heat the rate of production of oxygen becomes faster.
Balanced chemical equations for reversible reactions involving,

(a) Nitrogen as one of the reactants

(b) SO₂ as one of the reactants.

Question 8.
State which type of chemical reactions proceed with – (a) Evolution of heat energy (b) Absorption of heat energy. State in each of the following reactions whether heat is evolved or absorbed – (i) water is added to quicklime (ii) two neutral gases on passage through an electric arc give nitric oxide (iii) two neutral gases combine to give – a basic gas.
Answer:
(a) Exothermic reactions.
(b) Endothermic reactions.

1. When water is added to quicklime heat is evolved.
2. Nitrogen and oxygen.
3. Nitrogen gas and hydrogen gas give NH₃ (ammonia a basic gas).

Question 9.
Certain thermal decomposition reactions, result in formation of oxides. Give balanced equations for the thermal decomposition of the following, which result in formation of a metallic oxide

(a) Limestone

(b) Lead carbonate

(c) Calcium nitrate

(d) Calcium hydroxide.

Answer:

(a) Limestone CaCO₃ decomposes to CaO and CO₂
(b) Lead carbonate PbCO₃
(c) Calcium nitrate Ca[NO₃]₂
(d) Calcium hydroxide

Question 10.
State the meaning of the term ‘oxide’. Give a balanced equation for formation of the following oxides –

(a) Sulphur dioxide from a non-metal
(b) Zinc oxide from a metal
(c) Lead oxide from a mixed Oxide.

Answer:
Oxide is product obtained on combining with oxygen.
i. e. combination with oxygen.
OR
Binary compounds of a metallic or non-metallic element with oxygen.

Question 11.
Give two examples each of the following oxides – (a) Acidic oxides (b) Basic oxides (c) Amphoteric oxides (d) Neutral oxides. State which of the following oxides i.e. (a) to (d) (i) React with water to give a base (ii) React with a base to give salt & water (iii) React with acids & bases to give salt & water.
Answer:
(a) Acidic oxides : Non-metallic oxides are acidic oxides.

1. SO₂
2. CO₂
3. NO₂ are acidic oxides.

(b) Basic oxides : Metallic oxides are basic oxides.

1. Na20
2. K20
3. CaO are basic oxides.

(c) Amphoteric oxides : Oxides of metals like Zn, Pb, Al are amphoteric oxides.

1. ZnO
2. PbO
3. Al₂O₃ are amphoteric oxides.

(d) Neutral oxides : Oxides like NO,CO,N₂0 are neutral oxides.

1. Basic oxides react with water to give bases.
2. Acidic oxide reacts with base to give salt and water.
3. Amphoteric oxide reacts with acids and bases to give salt and water.

Question 12.
Give one example each of –

(a) A peroxide
(b) A mixed oxide
(c) A dioxide.

Answer:
Examples of :

(a) A peroxide — Na₂O₂ (Sodium peroxide)
(b) A mixed oxide — Pb₃0₄
(c) A dioxide – PbO₂

OBJECTIVE TYPE QUESTIONS

Q.1. Select the correct answer from A, B, C, D and E for each statement given below :
A: Iron
B: Carbonic acid
C: Hydrogen
D: Oxygen
E: Carbon monoxide

Question 1.
The product formed during direct combination reaction of carbon dioxide & water.
Answer:
B: Carbonic acid

Question 2.
The neutral gas obtained on thermal decomposition of potassium nitrate.
Answer:
D: Oxygen

Question 3.
The displaced product of the displacement reaction of sodium with cold water.
Answer:
C: Hydrogen

Question 4.
The catalyst used in the catalystic reaction involving the reactants nitrogen & hydrogen.
Answer:
A: Iron

Question 5.
A neutral oxide which does not react with an acid or a base to give salt & water.
Answer:
E: Carbon monoxide

Q.2. Complete the statements by filling in the blank with the correct word/s :

Question 1.
Direct combination reaction of phosphorus pentoxide with water gives ___ [H₃PO₃/H3PO₄].
Answer:
Direct combination reaction of phosphorus pentoxide with water gives H₃PO₄.

Question 2.
Decomposition of silver salts in the presence of sunlight is an example of ___ [double decomposition/photochemial decomposition].
Answer:
Decomposition of silver salts in the presence of sunlight is an example of  photochemial decomposition.

Question 3.
The element molybdenum used in the reaction of nitrogen with hydrogen at elevated temperatures is an example of a ___ [promoter/catalyst].
Answer:
The element molybdenum used in the reaction of nitrogen with hydrogen at elevated temperatures is an example of a promoter.

Question 4.
The reaction of coke with steam to give water gas is an example of an ___ [exothermic/endothermic] reaction.
Answer:
The reaction of coke with steam to give water gas is an example of an endothermic reaction.

Question 5.
The metal which reacts with steam and the reaction is reversible is ___ [calcium/iron].
Answer:
The metal which reacts with steam and the reaction is reversible is iron.

Q.3. Give a balanced equation for each of the following types of reactions :

Question 1.
A thermal decomposition reaction in which a compound decomposes to give two new compounds.
Answer:

Question 2.
A reaction of direct combination i.e. synthesis in which twp gases combine to give another gas – which turns lime wafer milky.
Answer:
Synthesis when two gases combine to give a gas which turns lime water milky.

Question 3.
A thermal decomposition reaction in which a metallic nitrate decomposes to give – a basic oxide.
Answer:
Metallic nitrate is calcium nitrate on thermal decomposition gives CaO which is a basic oxide.

Question 4.
A catalytic, reversible, exothermic reaction.
Answer:
A catalytic, reversible, exothermic reaction.

Question 5.
A displacement reaction in which a metal above hydrogen in the reactivity series, displaces another metal from the solution of its compound.
Answer:

Q.4. Differentiate between the following :

Question 1.
Thermal decomposition & thermal dissociation.
Answer:
Thermal decomposition and thermal dissociation.
Decomposition of a compound on heating into two elements or element and compound or into two new compounds. It is irreversible reaction.
Thermal dissocation is the decomposition of a substance into two or more simpler substances on heating but reaction is reversible.

Question 2.
Neutralization reaction & a precipitation reaction.
Answer:
Formation of salt and water when an acid reacts with a base is called neutralisation.

• Precipitation : Formation of semi solid (insoluble) product in called precipitation.
  

Question 3.
Electrolytic decomposition & photochemical decomposition.
Answer:

• Electrolytic decomposition : “A decomposition reaction which is brought about by – passage of electric current.”
  
• Photochemical decomposition : “A decomposition reaction which takes place in presence of light.”
  e.g. decomposition of silver salts in presence of sun light.

Question 4.
A catalyst & a promoter.
Answer:

• Catalyst : “A substance which alters the rate of chemical reaction.”
  e.g. MnO₂, V₂O₅, Fe, Pt. etc.
• The positive catalyst : A catalyst which speeds up the reaction.
• Negative catalyst : A catalyst which reduces the rate of reaction, e.g. alcohol.
  Enzymes are biological catalysts present in human body and accelerate the biochemical reactions in the body.
• Promotors is a catalyst to catalyst, i.e. a substance which increases the activity of a catalyst.
  Molybdenum [MO] is added to the catalyst iron [Fe] to increase its efficiency.

Question 5.
An acidic oxide & a basic oxide.
Answer:

• Acidic oxide : Non-metal oxide is called acidic oxide i.e. SO₂, NO₂ when acidic oxide dissolves in water forms acid.
• Basic oxide : Metallic oxide is called basic oxide, e.g. CaO, Na₂O basic oxide dissolved in water is called alkali.

Q.5. Match the chemical reactions in List I with the appropriate answer in List II.

Answer:

New Simplified Chemistry Class 8 ICSE Solutions – Language of Chemistry

ICSE SolutionsSelina ICSE SolutionsML Aggarwal Solutions

Simplified ChemistryChemistryPhysicsBiologyMathsGeographyHistory & Civics

EXERCISE

Question 1.
Explain the term ‘symbol’. State a reason why – the symbol of calcium is ‘Ca’ & of copper is ‘Cu’
Answer:
Symbol is a short form or abreviated name – of the element.
OR
“Symbol of an element is the ‘first letter’ or the ‘first letter and another letter’ of the English name or Latin name of the element”.
As the name calcium and copper start with same letter of English alphabet ‘C’ so another letter from the name of the element is added to ‘C’.
Hence symbol of calcium is ‘Ca’ and symbol of copper or cuprum (Latin name of copper) is ‘Cu’.

Question 2.
Define the term ‘valency’. With reference to water & ammonia as compounds respectively, state the valency of oxygen & nitrogen. Magnesium [2, 8, 2] has valency 2+. Give reasons.
Answer:
Valency is the number of hydrogen atoms which combine with (or displace) one atom of the element forming a Compound. Compound water H₂0, two atoms of hydrogen combine with one atom of oxygen to form water. Hence valency of oxygen atom is 2.
Compound ammonia NH₃.
Valency of atom nitrogen is 3, as it combines with 3 atoms of hydrogen.
Valency of magnesium is 2⁺, Mg [2, 8, 2]
Electronic configuration of Mg is 2, 8, 2 i.e. there are 2 valence electrons which Mg can lose and achieve stable configuration of nearest noble gas Neon.
Hence is cation Mg²⁺ has valency 2⁺.

Question 3.
Explain the term ‘variable valency’. Copper having electronic configuration 2,8,18,1 exhibits variable valency. Give a reason for the same & name the compound CuCI & CuCl₂.
Answer:
Variable valency : “When an element has more than one valency, its valency is called variable valency.”
Copper exhibit valency ‘1’ and ‘2’ i.e. has variable valency reason for variable valency :
Valency is – the number of electrons lost or gained from the outer shell of an atom of an element – during chemical reaction. Variation in this gain or loss results in ‘variable valency’.
Lower valency ends with – ous
Higher valency ends with – ic
CuCI is named as cuprous chloride (lower valency)
OR
Cu [I] Cl i.e. Copper [I] chloride and CuCl₂ – Cupric chloride (higher valency) i.e. Copper [II] chloride Cu[II]Cl₂

Question 4.
State the valencies of the following metallic elements –

(a) Potassium
(b) Sodium
(c) Calcium
(d) Magnesium
(e) Zinc
(f) Aluminium
(g) Chromium [write each symbol with the valency]

Answer:
Metals have positive valency 1, 2 or 3 ,
Valency of

(a) Potassium is K⁺
(b) Sodium 1⁺
Na⁺
(c) Calcium 2⁺
Ca²⁺
(d) Magnesium 2+
Mg²⁺
(e) Zinc 2⁺
Zn²⁺
(f) Aluminium 3⁺
Al³⁺
(g) Chromium 3+
Cr³⁺

Question 5.
Certain metals exhibit variable valencies which include valencies: 1+, 2+, 3+ & 4+.
State the variable valency of the following metals –

(a) Copper
(b) Silver
(c) Mercury
(d) Iron
(e) tin
(f) Lead

[write each symbol with the variable valency]

Answer:

Question 6.
State which of the following ions or radicals given below of non-metallic elements exhibit-valency: 1^–, 2^– & 3^–
(a) Chloride (b) Bromide (c) Iodide (d) Nitrate (e) Hydroxide (f) Bicarbonate (g) Bisulphite (h) Bisulphate (i) Aluminate (j) Permanganate (k) Oxide (l) Sulphide (m) Sulphite (n) Sulphate (o) Carbonate (p) Dichromate (q) Zincate (r) Plumbite (s) Phosphate (t) Nitride
[write each ion or radical with the correct valency]
Answer:

Question 7.
Differentiate between the terms – ‘Ion’ & ‘radical’ with suitable examples.
Answer:
Ion “is an atom or a group of atoms carrying a positive or negative charge due to loss or gain of electrons.” e.g. cation Na⁺ and Cl^1- is anion.
Radical “is group of atoms of elements that behaves like a single unit & show valency.”
Positive radical [NH H₄¹⁺ ] Ammonium
Negative radical [HCO₃^1- ], [NO₃^1-] Bicarbonate Nitrate

Question 8.
Write the chemical formula of the following compounds in a step-by-step manner – (a) Potassium chloride (b) Sodium bromide (c) Potassium nitrate (d) Calcium hydroxide (e) Calcium bicarbonate (f) Sodium bisulphate (g) Potassium sulphate (h) Zinc hydroxide (i) Potassium permanganate (j) Potassium dichromate (k) Aluminium hydroxide (l) Magnesium nitride (m) Sodium zincate (n) Copper [II] oxide (o) Copper [I] sulphide (p) Iron [III] chloride (q) Iron [II] hydroxide (r) Iron [III] sulphide (s) Iron [III] oxide.
Answer:

Question 9.
What’ is a chemical equation. How is it represented. Differentiate between a ‘word equation’ and a ‘molecular equation’ with a suitable example.
Answer:
“Chemical equation is a short form – representing the result of a chemical change.”
OR
“Is the representation of a chemical change.”
Word equation tells which substances react (take part) in chemical reaction and which substances are produced where as molecular equation symbols and molecular formulas are used for both reactants and products.
Example : When zinc reacts with dilute sulphuric acid, both being reactants, products produced are zinc sulphate and hydrogen gas, which are shown as below:
Word equation :
Zinc + Sulphuric acid → Zinc sulphate + Hydrogen
Molecular equation :

Question 10.
State the information provided by a chemical equation. Chemical equations suffer from a number of limitations. State the main limitations of a chemical equation.
Answer:
Information provided by a chemical equation :

(a) It tells us the formulas and symbols of the reactants and products.
(b) It tells us the ratio in which substances reacts or are produced. If limitations are covered.
(c) It tells the physical state of substances i.e. solid, liquid, gas.
(d) Whether the reaction is endothermic or exothermic.
(e) Conditions for starting the reaction i.e. if catalyst is needed or not.
(f) If reaction is reversible or not.

Limitations :

(a) Physical states of the reactants or products. But now we write along with substances (1) for liquids, (s) for solid and (g) for gas.
(b) Conditions that effect the reaction, i.e. temp, pressure or catalyst.
(c) Concentration of reactants and products we use dil. for dilute and cone, for concentrated.
(d) Nature of the chemical reaction.
(e) Speed – reaction is fast or slow.
(f) Exothermic or endothermic we write + heat or – heat towards products for exothermic and endothermic.
(g) The completion of the reaction.

Question 11.
State what is a balanced equation with a relevant example. Give a reason why an equation is balanced with reference to the law of conservation of matter.
Answer:
A balanced equation : “An equation is said to be balanced if the number of atoms of each element of the reactant is equal to the number of atoms of each element of the product.”
Example :
2Mg + O₂ → 2MgO
[Reactants] [Product]
Reactans have 2 atoms of Mg and 2 atoms of oxygen.
Products have 2 atoms of magnesium and 2 atoms of oxygen.
∴ Number of atoms of each element of reactants = Number of atoms of each element of product.

• Reason for balancing equation : Law of conservation of matter says that “Matter is neither created nor destroyed during a chemical reaction.” This is possible only if number of atoms of each element on both sides of → are equal i.e. in reactants and also in products. Hence a reaction is balanced.

Question 12.
Write balanced molecular equations for the following word equations :

Answer:

Question 13.
Balance the following equations :

Answer:

OBJECTIVE TYPE QUESTIONS

Q.1. Complete the statements given below by filling in the blank with the correct words.

Question 1.
The formula of silver [I] chloride is ___ [AgCl/AgCl₂].
Answer:
The formula of silver [I] chloride is AgCl.

Question 2.
The basic unit of an element is a/an ___ [molecule/atom/ion]
Answer:
The basic unit of an element is a/an atom.

Question 3.
Atoms contains ___ [netron/nucleus/, with positively charged __ [electrons/protons].
Answer:
Atoms contains nucleus with positively charged protons.

Question 4.
Element ___ [calcium/lead/carbon] has the symbol derived from its Laltin name ‘plumbum’.
Answer:
Element lead has the symbol derived from its Laltin name ‘plumbum’.

Question 5.
From the elements – He, Br, Pt & O; the element which forms a polyatomic molecule is ___ & which is liquid at room temperature is ___
Answer:
From the elements – He, Br, Pt & O; the element which forms a polyatomic molecule is O & which is liquid at room temperature is Br.

Question 6.
The valency of iron in FeO is ___ [2⁺/1⁺] of chlorine [chloride] in CaCl₂ is ___ [1^–/2^–] and of dichromate in K₂ Cr₂  O₇ is [2⁺/2^–].
Answer:
The valency of iron in FeO is 2⁺ of chlorine [chloride] in CaCl₂ is 1^– and of dichromate in K₂ Cr₂  O₇ is 2^–.

Q.2. Match the statements – 1 to 10 below with their correct answers from – A to J.

Answer:

Q.3. Match the compounds in List I – 1 to 20 with their correct formulas in List II – A to T.

Answer:

Q.4. Underline the incorrectly balanced compound in each equation & rewrite the correct equation.

Question 1.
2Na + 3H₂0 → 2NaOH + H₂
Answer:
Correct equation is :
2Na + 2H₂0 → 2NaOH + H₂

Question 2.
4P + 4O₂ → 2P₂O₅
Answer:
Correct equation is :
4P + 5O₂ → 2P₂O₅

Question 3.
FE₂O₃ + 2H₂ → 2Fe + 3H₂0
Answer:
Correct equation is :
FE₂O₃ + 3H₂ → 2Fe + 3H₂0

Question 4.
2A1 + 2H₂SO₄ → A1₂(SO₄)3 + 3H₂
Answer:
Correct equation is :
2A1 + 3H₂SO₄ → A1₂(SO₄)3 + 3H₂

Question 5.

Answer:
Correct equation is :

Question 6.
ZnO + 3NaOH → NA₂ZnO₂ + H₂0
Answer:
Correct equation is :
ZnO + 2NaOH → NA₂ZnO₂ + H₂0

Question 7.
FeCl₃ + 3NH₄OH → 2NH₄C1 + Fe(OH)₃
Answer:
Correct equation is :

Question 8.
FeS + 2HCl → 2FeCl₂ + H₂S
Answer:
Correct equation is :
FeS + 2HCl → FeCl₂ + H₂S

Question 9.
3NH₃ + H₂So₄ → (NH₄)₂So₄
Answer:
Correct equation is :
2NH₃ + H₂SO₄ → (NH₄)₂SO₄

Question 10.
PbO₂ + 4HCl → PbCl₂ + H₂0 + Cl₂
Answer:
Correct equation is :
PbO₂+ 4HCl → PbCl₂ + 2H₂0 + Cl₂

New Simplified Chemistry Class 8 ICSE Solutions – Atomic Structure

ICSE SolutionsSelina ICSE SolutionsML Aggarwal Solutions

Simplified ChemistryChemistryPhysicsBiologyMathsGeographyHistory & Civics

EXERCISE

Question 1.
State the main postulates of – Dalton’s atomic theory. Explain how the modern atomic theory contradicted Dalton’s atomic theory.
Answer:
(a) Dalton’s Atomic Theory :
The main postulates of theory are :

1. Matter consists of small indivisible particles called – atoms i. e. Atom is the basic unit of matter.
2. Atoms of the same element are – alike in all respects i.e. atoms of hydrogen have same properties like mass, density and atoms of oxygen are alike in all respects.
3. Atoms of different elements are different from each other. i.e. atoms of hydrogen are different from atoms of oxygen.
4. Atom can neither be created nor destroyed.
5. Atoms combine with other atoms in simple whole number ratio forming compounds or molecules.

(b) Modern atomic theory contradicted Dalton Atomic Theory as :

1. Atom is no longer indivisible as atom has been divided and has sub – atomic particles
   (a) Protons
   (b) Neutrons
   (c) Electrons
2. Atoms of same element may have different properties
   
3. Atoms of different elements may have same properties i.e. isobars
4. Atoms combine with other atoms may not be in simple whole number ratio i.e. C₁₂H₂₂O₁₁ (Sugar).
5. Atom can be destroyed and converted into energy.

Question 2.
With reference to the discovery of the structure of an atom, explain in brief – William Crookes experiment for the discovery of cathode rays, followed by – J.J. Thomsons experiment pertaining to the constituents of the cathode rays. State which sub-atomic particle was discovered from his experiment.
Answer:
Discovery of the three subatomic particles – electrons, protons and neutrons
Atom are built up of three sub-atomic particles – electrons, protons and neutrons.

Discovery of cathode rays leading to the discover of ‘electrons’

• Scientist – William Crookes [1878]
• Discovery – The cathode rays

Experiment :

1. An electric discharge was passed through a tube containing a gas at low pressure.
2. Blue rays were emitted from the cathode [negative plate] – which were called cathode rays.

Question 3.
Explain in brief – Goldstein’s experiment which led to the discovery of the proton and – Lod Rutherford’s experiment which led to the discovery of the atomic nucleus.
Answer:
Discovery of – Protons

• Discovery – Constituent of positive rays i.e. particles that contain – protons.

Experiment :

• Goldstein used a modified cathode ray tube with a perforated cathode.
• He observed a new type of rays produced from the anode passing through the holes of the perforated cathode. These rays were called anode rays.

Conclusion :

• Anode rays or positive rays consist of positively charged particles now called – protons.
• The positive rays were affected by electric & magnetic fields but – in a direction opposite to that of cathode rays.
• Thus with the discovery of the positive particles – proton was initiated.

Discovery of – Atomic nucleus :

• Discovery – Study of the atomic model leading to the discovery of – atomic nucleus.

Experiment :

• Rutherford projected alpha particles towards a thin gold foil, – in the path of the rays.
• He saw that most of the alpha particles went straight through the foil, – but some were deflected slightly & some by large angles.

Conclusion :

• An atom on the whole is relatively empty but consists of a – concentrated positive mass in the centre, which lead to the deflection of the alpha particles.
• Thus the discovery of a central positive region – atomic nucleus was initiated.

Question 4.
‘Electrons revolve around the nucleus in fixed orbits or shells called energy levels’. State how these energy levels are represented.
Answer:

(a) Electrons revolve around the nucleus in – fixed ‘orbits’ called energy levels
(b) The energy levels 1, 2, 3… are represented by – integer ‘n’ or as K, L, M, N…
(c) Electrons rotate around the nucleus, in one or .more of the energy levels.

Question 5.
Draw a neat labelled diagram representing an atom. Name the three sub-atomic particles in the atom & represent them symbolically showing the mass & charge of each. State where the sub-atomic particles are present in the atom.
Answer:

Question 6.
Define the term – ‘atomic number’ of an atom. If an atom ‘A’ has an atomic number of – eleven, state the number of protons & electrons it contains.
Answer:
Atomic number is the number of protons in the atom of an element. Since atom is electrically neutral i.e. is charge less, therefore
number of electrons = number of protons.
∴ It has 11 P and 11 electrons.
Atomic number z = p = e

Question 7.
Define the term – ‘mass number ’ of an atom. If an atom t ‘B’ has mass number 35 & atomic number 17, state the number of protons, electrons & neutrons it contains.
Answer:
Mass number of an element is equal to the sum of protons and neutrons in the nucleus of atom.
Mass number = Number of protons + Number of neutrons
A = p + n A = 35 atomic number p = 17
∴ 35 = 17 + n
∴ n = number of neutrons = 35 – 17 = 18
n = 18
But p = e
∴ e = p = 17
Number of electrons = e = 17

Question 8.
State why the atomic weight of an element is also termed – relative atomic mass.
Answer:

• Atomic weight : is mass of an atom, the number times it is heavier than an atom of hydrogen.
  Since carbon atom is 12 times heavier than an atom of hydrogen.
• Relative mass : is equal to the number of times an atom of an element is heavier than 1/12 the mass of an atom of carbon.
  Hence atomic weight of an element is also termed relative atomic mass as it is in comparison with mass of 1/12 th mass of a carbon atom.

Question 9.
State how electrons are distributed in an atom. Explain in brief the rules which govern their distribution.
Answer:
(a) Electrons revolve around the nucleus in imaginary paths called shells or orbits. Shells start from nucleus to outwards.

Rules : Maximum number of electrons in a shell is given by 2n². Where n is the number of shell i.e. 1st shell can have maximum of 2 electrons.
2n² = 2(1)² = 2 × 1 = 2
2nd shell can have maximum of 8 electrons
2n² = 2(2)² = 2 × 4 = 8
3rd shell can have maximum of 18 electrons
2n² = 2 (3)² = 2 × 9 = 18 and so on….
(b) Outer most orbit cannot have more than 8 electrons and 18 in penultimate orbit.
(c) A new shell cannot start until previous is filled completely.

Question 10.
If an atom ‘A’ has atomic number 19 & mass number 39, state –

1. Its electronic configuration.
2. The number of valence electrons it possesses.

Answer:
Atom ‘A’ has mass number A = 39 and atomic number Z = 19 = p
∴ A = Z + n
A = p + n
39 = 19 + n
n = 39 – 19 = 20
But e = p = 19

1. A (K, L, M, N)
   19 = 2, 8, 8, 1
   There will be 2 electrons in K-shell or 1st shell
   8 electrons in 2nd shell or L-shell
   8 electrons in 3rd shell or M-shell
   1 electron in 4th shell or nth-shell
2. The number of valence electrons i.e. in outer most shell = 1 electron.

Question 11.
Draw the atomic diagrams of the following elements showing the distribution of – protons, neutrons & the electrons in the various shells of the atoms.

[The upper number represents the – mass number & the lower number the – atomic number e.g. calcium – mass number = 40, atomic number = 20]
Answer:

Question 12.
‘ Valency is the number of hydrogen atoms which can combine with [or displace] one atom of the element [or radical] forming a compound’. With reference to the above definition of valency, state the valency of chlorine in hydrogen chloride, giving reasons.
Answer:
Hydrogen chloride [HCl], one atom of chlorine has combined with one atom of hydrogen and also 1 atom of hydrogen can be replaced by metals like potassium, sodium. Hence valency of chlorine in one.

Question 13.
‘ Valency is also the number of electrons – donated or accepted by an atom so as to achieve stable electronic configuration of the nearest noble gas’. With reference to this definition –

(a) State what is meant by ‘stable electronic configuration’.
(b) State why the valency of –

1. sodium, magnesium & aluminium is : +1, +2 & +3 respectively.
2. chlorine, oxygen & nitrogen is : -1, -2 & -3 respectively.

Answer:

(a) Stable electronic configuration means to have 2 electrons in the 1st [or K] outer most shell like He – [Duplet].
OR
8 electrons in outer most orbit like other nearst noble gas – [Octet].

(b)

1. Valency is the number of electrons donated or lost from the valence shell. Since sodium donates 1 valence electron its valency is +1.
   Magnesium loses 2 electrons and aluminium loses 3 electrons from their valence shell their valency is
   +2 – magnesium
   +3 – Aluminium
2. Valency of an element is the number of electrons accepted to achieve stable configuration of nearest noble gas.
   Chlorine accepts 1 electron and has valency -1 where as oxygen accepts 2 electrons the valency of oxygen is -2 and nitrogen accepts 3 electrons, valency of nitrogen is -3.

Question 14.
With reference to formation of compounds from atoms by electron transfer – electro valency, state the basic steps in the conversion of sodium & chlorine atoms to sodium & chloride ions leading to the formation of the compound – sodium chloride.
[electronic configuration of : Na = 2, 8, 1 & Cl = 2, 8, 7]
Answer:
Electronic configuration

OBJECTIVE TYPE QUESTIONS

Q.1. Match the statements in List I with the correct answer from List II.

Answer:

Q.2. Select the correct answer from the choice in bracket to complete each sentence :

Question 1.
An element ‘X has six electrons in its outer or valence shell. Its valency is __ [+2/-2/-1].
Answer:
An element ‘X has six electrons in its outer or valence shell. Its valency is -2.

Question 2.
An element ‘Y’ has electronic configuration 2, 8, 6. The element ‘Y’ is a __ [metal/non-metal/noble gas].
Answer:
An element ‘Y’ has electronic configuration 2, 8, 6. The element ‘Y’ is a non-metal.

Question 3.
A __ [proton/neutron] is a sub-atomic particle with no charge and unit mass.
Answer:
A neutron is a sub-atomic particle with no charge and unit mass.

Question 4.
An element Z with zero valency is a __ [metal/noble gas/non-metal].
Answer:
An element Z with zero valency is a noble gas.

Question 5.
Magnesium atom with electronic configuration 2, 8, 2 achieves stable electronic configuration by losing two electrons, thereby achieving stable electronic configuration of the nearest noble gas __ [neon/argon].
Answer:
Magnesium atom with electronic configuration 2, 8, 2 achieves stable electronic configuration by losing two electrons, thereby achieving stable electronic configuration of the nearest noble gas neon.

Q.3. The diagram represents an isotope of hydrogen [H]. Answer the following :

Question 1.
Are isotopes atoms of the same element or different elements.
Answer:
Isotopes atoms are of the same element.

Question 2.
Do isotopes have the same atomic number or the same mass number.
Answer:
Same atomic number.

Question 3.
If an isotope of ‘H’ has mass no. = 2, how many electrons does it have.
Answer:
One electron.

Question 4.
If an isotope of ‘H’ has mass no. = 3, how many neutrons does it have.
Answer:
Two neutrons. [∵ A = P + n]

Question 5.
Which sub-atomic particles in the 3 isotopes of ‘H’ are the same.
Answer:
Protons and electrons in each isotope are same.

Q.4. State the electronic configuration for each of the following :

Answer:
Electronic configuration of :

Q.5. Draw the structure of the following atoms showing the nucleus containing – protons, neutrons and the orbits with the respective electrons :

1. Lithium [At. no. = 3, Mass no. = 7]
2. Carbon [At. no. = 6, Mass no. = 12]
3. Silicon [At. no. = 14, Mass no. = 28]
4. Sodium [At. no. = 11, Mass no. = 23]
5. Isotopes of hydrogen [1₁H, 2₁H , 3₁H]

Answer:
Structure of atoms :
Z is Atomic Number A is mass number

 

New Simplified Chemistry Class 8 ICSE Solutions – Elements, Compounds & Mixtures

ICSE SolutionsSelina ICSE SolutionsML Aggarwal Solutions

Simplified ChemistryChemistryPhysicsBiologyMathsGeographyHistory & Civics

 

EXERCISE

Question 1.
Represent with the help of a simple chart how matter is classified into pure or impure substances & further into elements, compounds & mixtures, with elements further segmented.
Answer:

Question 2.
Define the terms elements, compounds & mixtures with a view to show their basic difference.
Answer:
Element is a pure substance.

1. It is the basic unit of matter and cannot be broken down into two or more simpler substances by any means.
2. It is mainly classified into metals, non-metals, metalloids and noble gases.

Compound is a pure substance.

1. It is formed by combination of two or more elements.
2. The elements are combined together in a fixed ratio.
3. It can be broken down into its elements by chemical means.

Mixture is an impure substance.

1. It is formed by combination of two or more elements, compounds or both.
2. The substances are mechanically mixed together in any ratio.

Question 3.
‘An atom is the basic unit of an element’. Draw a diagram of an atom – divisible as seen today.
Answer:
‘An atom is the basic unit of an element’.

Question 4.
‘The modern periodic table consists of elements arranged according to their increasing atomic numbers’. With reference to elements with atomic numbers 1 to 20 only in the periodic table – differentiate them into – metallic elements, metalloids, non-metals & noble gases.
Answer:
Names and symbols of metal, non-metals, metalloids and noble – gases out of 1st 20 elements.

Question 5.
Elements are broadly classified into metals & non-metals. State six general differences in physical properties of metals & non-metals. State two metals & two non-metals which contradict with the general physical properties – giving reasons. State one difference in property between metalloids & noble gases.
Answer:
Metallic elements :

(a) Have lustre – i.e. shine.
(b) Are malleable – i.e. can be beaten into sheets.
(c) Are ductile – i.e. can be drawn into wires.
(d) Are good conductors – of heat and electricity.
(e) have high – melting and boiling points.
(f) Have high – density.
(g) Contain – one type of atoms – monoatomic

Non-metallic Elements :

(a) Do no have lustre.
(b) Are non-malleable – i.e. cannot be beaten into sheets.
(c) Are non-ductile – i.e. cannot be drawn into wires.
(d) Are poor conductors – of heat and electricity.
(e) Have low – melting and boiling points.
(f) How low – density.
(g) Contain – monoatomic or diatomic atoms.

Two metals which contradict properties :

1. Mercury — is liquid at room temperature whereas metals are mostly solid.
2. Zinc — is NON-MALLEABLE and NON-DUCTILE contradicts metal which are MALLEABLE and DUCTILE.

Two Non-metals are :

1. Iodine — is lustrous contradicts non-metals have no lustre.
2. Graphite — is good conductor of electricity where as non – metals are poor conductor of electricity.

Question 6.
With reference to elements – define the term ‘molecule’. Give two examples each of a monoatomic, diatomic & polyatomic molecule.
Answer:
Molecule : Atoms of the same element or different elements combine to form a molecule. It is the smallest particle of a substance – which can normally exist independently and can retain, the physical and chemical properties of the substance.
Examples
Monoatomic molecule : Metals – Na, Mg etc.
Diatomic molecule : H₂, O₂, N₂.
Polyatomic molecule : O₃, P₄.

Question 7.
Define the term ‘compound’. In the compound carbon dioxide – the elements carbon & oxygen are combined in a fixed ratio. Explain.
Answer:
A compound is a pure substance made up of two or more different elements [atoms] combined chemically in a fixed proportion.

(a) Contains – two or more kinds of atoms, e.g. compound – carbon dioxide [CO₂].
(b) Can be broken down – into two or more simpler substances – by chemical means.
(c) Properties of compounds – differ from those of their elements, e.g. CO₂ contains elements – Carbon [C] is combustible and Oxygen [O] is supporter of combustion – but carbon dioxide is non-combustible and non-supporter of combustion.

Question 8.
State five different characteristics of compounds. Give three differences between elements & compounds with relevant examples.
Answer:
Five characteristics of a compound :

(a) Components in a compound are in a definite proportion.
(b) Compound is always homogenous [i.e. identical composition].
(c) Particles in a compound are of one kind.
(d) Compounds have a definite set of properties.
(e) Elements in the compound do not retain their original properties.
(f) Components in a compound can be separated by chemical means only.

Three differences between elements and compounds :
Element :

1. They contains one kind of atoms only e.g. element (Na) Sodium and [Cl] chlorine.
2. They elements have ther own set of properties e.g. [H] Hydrogen is combustible.
3. They cannot be broken down into two or more simpler substances by physical or chemical means.

Compound :

1. They contains two or more kinds of atoms, e.g. compound — water [H₂0].
2. They Properties of compounds are entirly new i.e. water [H₂0] is a liquid and can extinguish fire.
3. They Cannot be broken down into two or more simpler substances by chemical means.

Question 9.
Explain the term ‘mixture’. Differentiate between homogenous & heterogenous mixtures. State why brass is considered as a homogenous mixture while a mixture of iron & sulphur – heterogenous. Give an example of two liquids which form (a) homogenous (b) heterogenous – mixtures.
Answer:
Mixture : A mixture is made up of two or more substances elements or compounds or both mechanically mixed together in any proportion.
A mixture retain the properties of its constituent elements or compounds.

Differenance between homogenous & heterogenous mixtures :

Homogenous mixture :

• There constituents are uniformly mixed.
• The properties and composition are same throughout the mixture
• Two solids are Brass [Cu + Zn]

Heterogeneous mixture :

• There constituents are not uniformly mixed.
• The properties and composition vary throughout the mixture.
• Two solids are Iron + sulphur

BRASS is HOMOGENOUS because mixture is just the same through out and its constituents cannot be distinguished from each other i.e. composition is same through out.
Whereas in Heterogenous mixture of iron and sulphur, the constituents. Can be separated easily by a magnet, the particles can be distinguished from each other and composition is not uniform.

(a) Two liquids which form Homogenous mixture are Alcohol and water.
(b) Two liquids which form Heterogenous mixture are oil and water.

Question 10.
Compare the properties of iron [II] sulphide with iron – sulphur mixture, considering iron [II] sulphide as a compound & particles of iron & sulphur mixed together as an example of a mixture.
Answer:
Iron and sulphur mixture :

1. The constituents i.e. iron and sulphur are seen separately.
2. Iron attracts and clings to magnet.
3. On adding dil. HC1 to the mixture H₂ gas evolves.

Iron [II] sulphide is a compound

1. It is black solid.
2. On bringing a magnet near it we cannot separate iron.
3. On adding dil. HCl to it H₂S gas evolves.

Question 11.
State any one method – to separate the following mixtures –

(a) Two solid mixtures one of which – directly changes into vapour on heating.
(b) Two solid mixtures one of which – dissolves in a – particular solvent and other does not
(c) A solid-liquid mixture containing – an insoluble solid in the liquid component
(d) A solid-liquid mixture containing – a soluble solid in the liquid component
(e) A liquid-liquid mixture containing – two immiscible ”’ liquids having different densities
(f) A liquid-liquid mixture containing – two miscible liquids having different boiling points.
(g) A liquid-gas mixture containing – a gas dissolved in a liquid component.
(h) A gas-gas mixture containing – two gases with different densities.
(i) A mixture of different solid constituents – in a liquid constituent.

Answer:

(a) The method used is sublimation : Sublimable solid sublimes on heating i.e. changes into vapours and condenses on cooling is separated leaving behind non – sublimable solid.
(b) The method is solvent extraction : Soluble solid dissolves in solvent leaving behind the other insoluble solid. The dissolved solid is recovered by evaporation.
(c) The method is filtration : The insoluble solid is obtained on the filter paper as residue.
(d) The method is evaporation or distillation : Liquid evaporates leaving behind solid.
(e) The method is separating funnel : Heavier liquid collects in die flask below an opening tap, while lighter layer remains in the separating funnel.
(f) Method is fractional distillation : The liquid with lower boiling point collects in the receiver while the liquid with higher B.P. remains in distillation flask.
(g) The method used is boiling the mixture : Solubility of gas decreases with increase in temperature, so gas escapes when mixture is boiled and collected separately.
(h) The method is diffusion : The lighter gas diffuses more rapidly on passing the rough porous partition where as heavier gas diffuses less rapidly on passing through porous partition.
(i) The method used is chromatography.

Question 12.
Explain with diagrams the process used to – separate the following substances from the given mixtures.

(a) Ammonium chloride from a mixture of – ammonium chloride & potassium chloride.
(b) Iron from a mixture of – iron & copper
(c) Sulphur from a mixture of – sulphur & copper.
(d) Potassium nitrate from a mixture of – potassium nitrate & potassium chlorate.
(e) Lead carbonate [insoluble] from a mixture of – lead carbonate & water.
(f) Lead nitrate [soluble] from a mixture of – lead nitrate & water Le. lead nitrate solution.
(g) Carbon tetrachloride from a mixture of – carbon tetrachloride [heavier component] & water.
(h) Benzene from a mixture of – benezene [b.p. 80°C] & toluene [b.p. 110°C].
(i) Different dyes – in their liquid constituent ink.

Answer:

(a) By sublimation : On heating the mixture in evaporating dish, ammonium chloride sublimes on the walls of funnel and potassium chloride remains in evaporating dish.

Here A is Ammonium Chloride and B is Potassium Chloride.
(b) By magnetic separation : By bringing a magnet near the mixture iron pieces can be separated which will cling to the magnet.

Here A is Iron and B is Copper.
(c) By solvent extraction : Mixture of copper and sulphur is added to the beaker containing solvent carbon disulphide and stirred well. Sulphur dissolves. Put this mixture on filter paper in the funnel. Copper remains on filter paper and sulphur passes into the beaker as filtrate. Sulphur separates as carbon disulphide evaporates.

Here A is for Copper and B is Sulphur.
(d) Potassium nitrate KNO₃ is more-soluble than potassium chlorate KClO₃.
On heating to get saturated solution and on cooling the saturated solution less soluble (KClO₃) crystallise out. .More soluble KNO₃ is filtered out from hot saturated solution and is recrystallised from hot water and dried.

(e) Evaporation : Lead carbonate can be separated by evaporation. On evaporation, water evaporates leaving behind solid lead carbonate which has higher M.P.

(f) Lead nitrate is separated from soluble lead nitrate solution by crystallisation.

(g) By separing funnel, heavier CCl₄ carbon tetrachloride form the lower layer is separated when tap is opened and is collected in the flask. Water the lighter top layer remains in the funnel.

(h) By fractional distillation, miscible low boiling point benzene (B.P. 80°C) evaporates on heating the mixture and condenses in and collects in flask ‘Y’ where as higher boiling pt. Toluene (B.P. 110°C) remains in flask ‘X’ after condensation.

(i) By chromatography : Different dyes [solid constituents i.e. A, B, C, D] in ink which is the liquid constituent. By placing the ink spot containing different solid constituents [dyes] on the filter paper. Filter paper is hung with it’s lower end completely dipped in the solvent.

The solvent flows over the ink spot and the solid constituents [dyes ‘A’, ‘B’, ‘C’, ‘D’] separate out.

OBJECTIVE TYPE QUESTIONS

Q.1. Select the correct answer from A, B, C, D & E for each statement given below :
A: Gunpowder
B: Iodine
C: Boron
D: Helium
E: Bromine

Question 1.
A diatomic molecule.
Answer:
E: Bromine

Question 2.
A metalloid.
Answer:
C: Boron

Question 3.
A non-metal which is lustrous.
Answer:
B: Iodine

Question 4.
A mixture consisting of elements & a compound.
Answer:
A: Gunpowder

Question 5.
A noble gas.
Answer:
D: Helium

Q.2. Match the separation of components in List I with the most appropriate process in List II.

Answer:

Q.3. The diagram represents fractional distillation for separation of mixtures. Answer the following :

Question 1.
Can two immiscible liquids be separated by this process.
Answer:
No, they can be separated by separating funnel.

Question 2.
Separation of liquids by this process is based on which physical property?
Answer:
The physical property on which separation is based “Difference in their boiling points.”

Question 3.
If methyl alcohol & water are to he separated, which liquid would remain in flask ‘X’ after condensation.
Answer:
Water having boiling point 100°C will remain in flask ‘X’.

Question 4.
Give a reason for the above answer.
Answer:
Alcohol having lower B.P. 78°C will evaporate first and condense in the conical flask — receiver ‘Y’.

Question 5.
State the purpose of the fractionating column in the apparatus.
Answer:
The upper part of the ‘Fractionating column is cooler, so as the hot vapours rise up in the column, they get cooled (condense) and trickle back into the distillation flask ‘X’.

Q.4. Select the correct answer from the choice in bracket to complete each sentence :

Question 1.
Dust in air is an example of __ [heterogeneous / homogenous] mixture.
Answer:
Dust in air is an example of Homogenous mixture.

Question 2.
A soluble solid is separated from an insoluble solid by __ [fractional crystallisation / solvent extraction].
Answer:
A soluble solid is separated from an insoluble solid by solvent extraction.

Question 3.
The reactive element from the two monoatomic elements is __ [neon/silicon].
Answer:
The reactive element from the two monoatomic elements is silicon.

Question 4.
Compounds are __ [homogenous or heterogenous / always homogenous] in nature.
Answer:
Compounds are always homogenous in nature.

Question 5.
An example of a monoatomic molecule is __ [hydrogen / helium],
Answer:
An example of a monoatomic molecule is helium.

Q.5. Give reasons for the following statements :

Question 1.
Components in a mixture can be separated by physical methods only.
Answer:
Components of a mixture can be separated by physical methods because particles remain separate without chemical reaction between them.

Question 2.
Centrifugation can be used for separating an insoluble heavier solid, present in an – insoluble solid-liquid mixture.
Answer:
Centrifugation is fast method to separate suspended (heavier) insoluble solid from lighter liquid by rotating the mixture fast. Heavier solid settles down at the bottom.

Question 3.
The filter paper made into a cone & placed in a funnel for filtering out the solid particles in a solid-liquid mixture, should be moistened before placing.
Answer:
Moistening the filter paper cone sticks to the walls of the funnel and also makes filtration convenient.

Question 4.
Brass & bronze are examples of mixtures, while copper sulphate & lead nitrate are examples of compounds.
Answer:
Brass and bronze are mixtures as the composition of elements is not fixed by mass and have no formula.
Lead nitrate and copper sulphate are compounds as they have fixed ratio of elements by mass and have formula like Pb[NO₃]₂ and CuSO₄.

Question 5.
Zinc is considered an element, while zinc sulphide is considered a compound.
Answer:
Zinc [Zn] is an element as zinc is a pure substance made up of one kind of atoms all having same size, atomic number and atomic mass.
Zinc sulphide [ZnS] is pure substance made up of two element [atoms] of zinc and sulphur combined chemically in a fixed proportion.