objective workbook for simplified icse chemistry class 8

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.

New Simplified Chemistry Class 8 ICSE Solutions – Physical & Chemical Changes

ICSE SolutionsSelina ICSE SolutionsML Aggarwal Solutions

Simplified ChemistryChemistryPhysicsBiologyMathsGeographyHistory & Civics

EXERCISE

Question 1.
Change – is the law of nature which occurs in everyday life.
State when a substance [Le. matter] undergoes a kind of change.
Answer:
Change is the law of nature which occurs in everyday life. A substance [i.e. matter] undergoes a kind of change when- subjected to energy changes.
Matter is said to undergo certain changes when energy is added to the matter or removed from the matter.
Examples of Changes are :

1. Ice melts on supplying heat to it.
2. Milk changes into curd when a little curd is added to hot milk.

Question 2.
Give a reason why

(a) Freezing of water is a reversible change while burning of a candle – an irreversible change,
(b) Change of seasons is a periodic change while change of weather is a non-periodic change.

Answer:

(a) Freezing of water is reversible change as on heating frozen ice changes to water by heating.
Burning of a candle is irreversible change as on reversing the conditions we can not get candle back out of CO₂, water vapours and wax vapours.
(b) Change of season is periodic change as it occurs at regular intervals. Change of weather is non-periodic change because, it does not occur periodically at regular interval. Weather can change suddenly and it may rain in the evening.

Question 3.
State which of the following pertain to – chemical changes.

(a) Substance retains its identity
(b) The change is permanent
(c) No new substance is formed
(d) Is easily reversible
(e) Produces one or more new substances by change in its composition
(f) The change is temporary
(g) The composition and properties of the original substance are not altered.

Answer:
Chemical change :

(b) The change is permanent.
(e) Produces one or more new substances by change in its composition.
(g) The composition and properties of the original substance care not altered.

Question 4.
Give four reasons why burning of a magnesium ribbon in air is considered a chemical change.
Answer:
BURNING OF MAGNESIUM RIBBON is chemical change as : It is

1. Irreversible change
2. New product with new properties is formed (MgO)
3. It is permanent change
4. Composition is changed
5. Energy is required to bring the change.

Question 5.
Compare the energy changes involved during a physical and a chemical change.
Answer:

• Physical change : No gain or loss of energy during the physical change.
  Energy required for completion of a physical change is released when the change is reversed.
• Chemical change : Energy is either absorbed or evolved during the chemical change.
  Energy in the form of light, percussion (pressure) or generally heat energy is required for a chemical change.

Question 6.
Is change of state of matter – a physical or a chemical change. Give reasons.
Answer:
Change of state of matter is a PHYSICAL change since on reversing the conditions, original state can be attained.

Question 7.
State which of the following terms connected with change of state of matter ie.

(a) Melting
(b) Vaporisation
(c) Condensation
(d) Freezing

pertain to –

1. Conversion of a liquid into a vapour or gas
2. Conversion of a liquid into a solid.
3. Conversion of a solid into a liquid on heating.

Answer:

1. (b) Vaporisation
2. (d) Freezing
3. (a) Melting

Question 8.
Potassium chloride is added to water and stirred. A salt solution is obtained which is then boiled leaving behind a residue.

(a) Is the above change physical or chemical
(b) Name the residue which remains behind after the salt solution is boiled.
(c) Is the change reversible or not reversible.
(d) Are the composition and properties of the original substance altered.
(e) Give a reason why the above experimentation would not be possible, if calcium carbonate is taken, in place of potassium chloride.

Answer:

(a) Physical change.
(b) White residue of potassium chloride.
(c) Change is reversible.
(d) No, the composition and properties are not altered.
(e) When in place of KC1, we take calcium carbonate the above experiment cannot be possible as no new substance with new properties is produced, since CaCo₃ is insoluble in water. Boiling will also not work as CaCo₃ is in solid form in water.

Question 9.
Give a reason why –

(a) Heating a platinum wire is a reversible change but, heating a magnesium wire is an irreversible change.
(b) Addition of zinc nitrate to water is a physical change but, addition of zinc to dilute nitric acid is a chemical change.
(c) Rusting of iron is a chemical change but, magnetization of iron is a physical change.

Answer:

(a) Heating a platinum wire, final product is again platinum wire (as platinum is noble metal and does not react with air), i.e. no change in composition and is physical change. Heating magnesium wire forms MgO i.e. composition changes a new substance white powder of magnesium oxide is formed is chemical change.
(b) Zinc nitrate (soluble in water) disolves in water and is reversible. No change in composition takes place is a physical change where as zinc reacts with dil. nitfic acid to produce H₂ gas which gets oxidized to water by nitric acid (oxidizing agent) and hence is a chemical change.
(c) Rusting of iron is a chemical change as composition changes, reddish brown flaky powder is formed with new properties and is irreversible, permanent change but magnetization of iron is temporary and reversible change with no change in composition is a physical change.

Question 10.
Select the chemical changes from the following list of changes

(a) Vaporisation of water into water vapour.
(b) Boiling of milk.
(c) Respiration in mammals.
(d) Rotting of eggs.
(e) Drying of a fruit
(f) The carbon cycle.
(g) Occurrence of lightening.
(h) Breaking of glass.
(i) Butter turning rancid.
(j) Glowing of an electric bulb.
(k) Crystallisation of a salt from its solution.
(l) Change of seasons.
(m) Preparation of carbon dioxide from calcium carbonate & dilute hydrochloric acid.

Answer:

(a) Change of water into water vapours is a PHYSICAL change as no change in composition, only state changes, is reversible change on cooling vapour water is formed.
(b) Boiling milk is PHYSICAL change as no change in composition, temporary and reversible, no new substance with new properties is formed.
(c) Respiration in mammals is a CHEMICAL change, we breathe in oxygen and breathe out carbon dioxide. It is irreversible and composition changes.
(d) Rotting of eggs is CHEMICAL change, change in composition takes place, is irreversible process.
(e) Drying of a fruit is PHYSICAL change as composition remains same and only state changes, no new substance with new properties is produced.
(f) Carbon cycle is a PHYSICAL change as carbon remains as carbon, mass of carbon remains same in the cycle, is temporary and only change in state takes place.
(g) Occurance of lightening is a CHEMICAL change, energy is produced, new substance with new properties is produced.
(h) Breaking of glass is a PHYSICAL change as no change in composition takes place, no new substance is formed.
(i) Butter turning rancid is a CHEMICAL change as composition changes, new substance with new properties is formed, irreversible and permanent change.
(j) Glowing of an electric bulb is a PHYSICAL change as no change in composition of filament takes place is reversible, only filament glows when electricity is passed and returns to its original form on switching off the electric current.
(k) Crystallisation of a salt from its solution is a PHYSICAL change as only state changes (crystals are formed), composition does not change, is reversible, no change in mass takes place.
(l) Change of season is a PHYSICAL change, only change in temperature is seen and is reversible change.
(m) Preparation of carbondioxide from calcium carbonate and dilute hydrochloric acid is a CHEMICAL change as new K substance with new properties is formed, the change is permanent and irreversible.

OBJECTIVE TYPE QUESTIONS

Q.1. Select the correct answer from the choice in bracket to complete each sentence

Question 1.
A change in which a substance retains its identity is a __ [physical/chemical] change.
Answer:
A change in which a substance retains its identity is a physical change.

Question 2.
Matter is added or removed during a __ [chemical/ physical] change.
Answer:
Matter is added or removed during a chemical change.

Question 3.
Energy required for completion of a physical change is __ [released/absorbed] when the change is reversed.
Answer:
Energy required for completion of a physical change is released when the change is reversed.

Question 4.
Dissolution of lead nitrate in water is deemed as a __ [physical/chemical] change.
Answer:
Dissolution of lead nitrate in water is deemed as a physical change.

Question 5.
A change in which matter undergoes a change but the total mass of substance is unaltered is a __ [physical/ chemical] change.
Answer:
A change in which matter undergoes a change but the total mass of substance is unaltered is a chemical change.

Q.2. State which of the following 1 to 5 pertain to – A: Physical Change B: Chemical change :

1. Sublimation.
2. Fermentation.
3. Liquefaction or condensation.
4. Magnetization.
5. Respiration.

Answer:

1. Sublimation – A : Physical Change
2. Fermentation – B : Chemical change
3. Liquefaction or condensation – A : Physical Change
4. Magnetization – A : Physical Change

Q.3. Give reasons why the following are considered as chemical changes.

Question 1.
Copper carbonate on heat gives copper oxide and carbon dioxide.
Answer:
Copper carbonate [CuCO₃] green colour on heating gives copper oxide [Black colour CuO] and CO₂.
Hence composition is changed new substances CuO [Black] and CO₂ with new properties are produced and it is irreversible reaction hence it is a chemical change.

Question 2.
A bright light is seen evolved when a strip of magnesium is heated.
Answer:
Magnesium is heated producing a bright light to form MgO in air is a chemical change as the composition changes (MgO white powder) is formed, is irreversible. We cannot get magnesium back from MgO.

Question 3.
Sulphur when burnt in air evolves a gaseous acidic product.
Answer:
Is a chemical change as sulphur (yellowish green) on burning produces SO₂ gas which is acidic in nature and a new substance with new properties is formed.

Question 4.
An iron nail kept open in the atmosphere rusts.
Answer:
Rust of iron is a chemical change as iron rust [Fe₂ O_{3 χ} H₂0] is brown flaky powder having different composition from original iron metal, is irreversible change.

Question 5.
A piece of magnesium strip is dropped into a beaker containing dilute hydrochloric acid.
Answer:
When a piece of magnesium strip is dropped in dilute hydrochloric acid, hydrogen gas is evolved and hence it is a chemical change.

as new substances MgCl₂ and H₂ with new properties are produced and the reaction is irreversible.

Q.4. Match the examples of changes in List I with the correct type of change in List II.

Answer:

Q.5. Name the following.

Question 1.
The salt obtained when a chemical change takes place on addition of iron to dilute sulphuric acid.
Answer:
FeSO₄ (Iron Sulphate)

Question 2.
The product obtained during a physical change when water converts from a liquid into a solid.
Answer:
Ice.

Question 3.
The product of the chemical change on keeping a polished iron nail exposed to the atmosphere.
Answer:
Iron III oxide [Fe₂O₃x H₂0]

Question 4.
A form of energy required for a chemical change.
Answer:
Generally HEAT ENERGY is required but light, pressure or electricity can also be needed.

Question 5.
The gaseous product of the chemcial change which takes place during respiration in living organisms.
Answer:
CO₂ carbon dioxide.

New Simplified Chemistry Class 8 ICSE Solutions – Matter

ICSE SolutionsSelina ICSE SolutionsML Aggarwal Solutions

Simplified ChemistryChemistryPhysicsBiologyMathsGeographyHistory & Civics

EXERCISE

Question 1.
Explain the meaning of the term ‘matter’.
Answer:
Matter is defined as anything that has mass, occupies space and can be felt by the senses.

Question 2.
Matter in any state is composed of small particles – molecules, atoms or ions. Differentiate the terms above in Italics.
Answer:
Molecules : A molecule is the smallest particle of a substance that can normally exist separately and retain the characteristics of the substance, e.g. O₂, H₂, Cl₂ are molecules.
Atoms : An atom is the smallest particle of an element which can take part in a reaction and is considered the basic unit of matter, e.g. O, H, Cl are atoms.
Ions : An ion is any atom or a group of atoms which has a resultant charge due to loss or gain of electrons, e.g. O^2-, H¹⁺, Cl^1-– are ions.

Question 3.
Differentiate between the-two characteristics of matter – ‘mass’ & ‘weight’.
Answer:
Mass :

1. The amount of matter in a body.
2. It is measured in Kilograms (kg).

Weight :

1. Due to the pull of gravity on a body.
2. It is measured in Newtons (N)

Question 4.
State which of the three states of matter Le. solids, liquids or gases – have

(a) No definite volume
(b) A definite shape
(c) High density
(d) No free surfaces
(e) Particles – which diffuse very easily.

Answer:

(a) Gases
(b) Solids
(c) Solids
(d) Gases
(e) Gases

Question 5.
State the main postulates of the kinetic theory with special reference to –

(a) Inter-particle space
(b) Inter-particle attraction
(c) Energy possessed by particles of matter.

Answer:

(a) Inter-particle space : The particles are arranged in a way such that they have spaces existing between them. i.e. inter molecular space.
(b) Inter-particle attraction : All particles attract each other with a force which is maximum if the particles are close to each other and minimum if the particles are at a distance from each other i.e. decreases with increase distance and vice-versa.
(c) Energy possessed by particles of matter : Since particles are in continuous motion, they, possess energy called kinetic energy. Application of heat increases kinetic energy of particles – which then move more randomly.

Question 6.
State in which of the following examples ie. a piece of wood, water, a light gas is the

(a) Inter-particle space maximum
(b) Inter-particle attraction maximum
(c) Energy possessed by particles of matter, very large.

Answer:

(a) A light gas
(b) a piece of wood
(c) a light gas

Question 7.
In which of the three states of matter – solids, liquids or gases is the movement of atoms about their own position. Give a reason for the same.
Answer:
Movement of atoms[molecules] in solids are about their own position.

This is because of Minimum Inter Particle space and very strong inter – particle attraction among atoms.

Question 8.
‘Inter-particle attraction between atoms of gases is very weak’. State five properties of gases which correlate as a consequence of the weak inter-particle attraction between particles of gases.
Answer:
Gases are

1. least rigid
2. highly compressible

Inter-Particles attraction between atoms of gases is very weak. The five properties of gases which correlate as a consequence of the weak inter-particle attraction between particles of gases are :

• They have no definite volume.
• They have no definite shape.
• They have no free surface.

Gases Rapidly diffuse with other gas particles,

Question 9.
What is inter-conversion of matter. Give the meaning of the terms involved in inter-conversion of matter

(a) Melting
(b) Vaporisation
(c) Liquefaction or condensation,
(d) Solidification or freezing
(e) Sublimation.

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.”

(a) MELTING : Conversion of a solid into a liquid on heating e.g. ice to water.
(b) VAPORISATION : Conversion of a – liquid into vapours (or gas) on heating e.g. water to water vapour.
(c) LIQUEFACTION or CONDENSATION : Conversion of VAPOUR (or gas) into a LIQUID on cooling e.g. water vapour to water.
(d) SOLIDIFICATION OR FREEZING : Conversion of a LIQUID into a SOLID e.g. water to ice.
(e) SUBLIMATION : Conversion of a — SOLID directly in GASEOUS (vapour) state and on cooling directly back to SOLID e.g. Naphthalene sublimes at room temperature.

Question 10.
Draw a labelled schematic diagram representing the terms (a) to (e) involved in the inter-conversion of matter.
Answer:
(a) Melting

(b) Vaporisation

(c) Liquefaction or condensation

(d) Solidification or freezing

(e) Sublimation

Question 11.
With reference to inter-conversion of matter – on the basis of kinetic theory – explain in brief the conversion of :

(a) A solid into a liquid
(b) A liquid into vapour [or gas]
(c) Vapour [or gas] into a liquid
(d) A liquid into a solid

With special reference to inter-particle space & inter particle attraction at the different stages of conversion.
Answer:
(a) A solid into a liquid : Kinetic energy increases

1. Heat energy is absorbed by the molecules and stored in the form of potential energy.
2. Inter-particle space-increases.
3. Inter-particle attraction-decreases.
4. Collision-increases and push apart.
   Solid changes into liquid.

(b) A liquid into vapour [or gas] :

1. Heat energy absorbed -stored as potential energy.
2. Inter-particle space-increases.
3. Inter-particle attraction-decreases (almost negligible).
4. Collision-increase and particles push apart become free and escapes gas.
   Liquid changes into vapours.

(c) Vapour [or gas] into a liquid :

1. Potential energy released in the form of heat energy.
2. Inter-particle space-decreases considerably.
3. Inter-particle attraction-increases.
4. Collision-decrease to negligible particles are not in a position to be free.
   Vapours changes into liquid state.

(d) A liquid into a solid :

1. Potential energy (stored) is-released.
2. Inter-particle space-decreases.
3. Inter-particle attraction-increases. Particles are not in position to be free.
4. Collision-decreases.
   Liquid changes into solid.

Question 12.
On the basis of kinetic theory explain why, ammonium chloride sublims and goes from solid state directly into vapour state.
Answer:
Ammonium Chloride sublimes and goes from solid state directly into vapour state because if low inter-particle attraction.
Sublimable solids have very low force of attraction between their particles.

Question 13.
State the ‘law of conservation of mass’. State the main points of Landolt’s experiment for experimental evidence of the law.
Answer:
Law of Conservation of Mass : “In any chemical reaction, the total mass of the reacting substances is equal to the total mass of the products of the reaction provided masses are measured under similar conditions.”
Landolt’s experiment : To illustrate the law of conservation of mass. Two solutions – NaCl in limb A and AgNO₃ sol. in limb B were taken in U-shaped tube and weighed.

Now tube was tilted so that two solutions get mixed and react with each other to form new products. Tube is weighed again
Weight after reaction was found to be same as before reaction.

Total mass of reactants = Total mass of products This verifies the law.
Two other examples are :

1. We can take two solutions of lead acetate Pb (CH₃00)₂ in limb A and sodium sulphate Na₂So₄ in limb B in the U-shaped tube.
   Pb(CH₃COO)₂ + Na₃So₄ → PbSo₄ + 2CH₃COONa
2. We can also take two solutions of iron [II] sulphate FeSo₄ in limb A and silver sulphate Ag₂So₄ in limb B in U- shaped tube still the result is found to be same.
   2FeSo₄ + Ag₂So₄ → 2Ag ↓ + Fe₂(So₄)₃

OBJECTIVE TYPE QUESTIONS

Q.1. Select the correct answer from A, B, C, D & E for each statement given below:
A: Solid
B: Vaporization
C: Ion
D: Gases
E: Heat

Question 1.
An atom or group of atoms – having a resultant charge.
Answer:
C : Ion

Question 2.
The state of matter which has – least density & no free surfaces.
Answer:
D : Gases

Question 3.
In Landolt’s experiment – the form in which the chemical energy stored up in the reactants – is released.
Answer:
E : Heat

Question 4.
The process of change of a liquid into vapour, [gas] on heating.
Answer:
B : Vaporization

Question 5.
The state of matter, where the inter-particle attraction between particles is maximum.
Answer:
A : Solid

Q.2. With reference to the liquid state of matter – answer the following

Question 1.
On heating the liquid, do the particles gain or lose energy.
Answer:
Gain energy.

Question 2.
The heat energy supplied to the liquid is absorbed by its molecule & stored as which form of energy.
Answer:
Potential energy.

Question 3.
How does the stored energy, have effect on inter-particles space.
Answer:
Stored potential energy increases the space.

Question 4.
State why the inter-particles attraction decreases to negligible.
Answer:
This is so because when distance between particles increases, and force of attraction decreases leading to Inter-particle attraction to almost negligible.

Question 5.
At what point will the particles become free and escape as gas.
Answer:
At boiling point.

Q.3. Complete the statement by filling the blanks with the correct word from the brackets.

Question 1.
In naphthalene, the inter-particle attraction is __ [high/low].
Answer:
In naphthalene, the inter-particle attraction is low.

Question 2.
The law of conservation of mass, is strictly valid if mass and __ [energy/volume] are considered together.
Answer:
The law of conservation of mass, is strictly valid if mass and energy are considered together.

Question 3.
When inter-particle space increases, the inter-particle attractive force __ [decreases/increases].
Answer:
When inter-particle space increases, the inter-particle attractive force decreases.

Question 4.
Kinetic energy of molecules in helium is __ [large / very large] compared to the kinetic energy of molecules in water.
Answer:
Kinetic energy of molecules in helium is large compared to the kinetic energy of molecules in water.

Question 5.
Conversion of vapour [or gas] into a liquid is termed __ [liquefaction/vaporization].
Answer:
Conversion of vapour [or gas] into a liquid is termed liquefaction.

Q.4. Give reasons for the following 

Question 1.
Particles of matter possess energy called kinetic energy.
Answer:
Particles of matter are contiunously moving, that is, they possesses kinetic energy.

Question 2.
Solids cannot be compressed, but gases are highly compressible.
Answer:

• In solids : Particles are closely packed. Inter-particle space is minimum and hence cannot be compressed.
• In gases : Particles are least compact and inter-particle spaces are maximum. Hence are highly compressible.

Question 3.
Kinetic energy of molecules of gases is very large & of solids, the least.
Answer:
The Kinetic Energy of molecules of gases is very large because :

1. The inter-particle space between the particles is maximum.
2. The Inter-particle attraction between the particles is negligible. Whereas The kinetic energy of molecules of solids is least

because :

1. The Inter-particle space between the particles is maximum.
2. The Inter-particle attraction between the particles is very strong.

Question 4.
On heating a sublimable solid, the molecules break free & escape from surface of the solid directly into vapour.
Answer:
On heating a sublimable solid the inter-particle attraction is overcome and the molecules break-free and escapes from the surface of the solid directly into the vapour.

Question 5.
Particles of matter move more rapidly on application of heat.
Answer:
Application of force increases the K.E. of particles and space increases and inter-particle attraction decreases. Thus particles
move rapidly.

Q.5. Complete the blanks with reference to interconversion of matter on basis of kinetic theory-with the word ‘increase’, ‘decreases’, ‘gain’, ‘lose’ or ‘overcome’ in each case.

1. During melting of solids, the inter-particles space increases.
2. During vaporization, the liquid particles gain energy.
3. During liquefaction, the particles lose energy.
4. During soldification, the inter-particle space decreases gently.
5. During sublimation the inter-particle attraction is overcome.