Selina ICSE Chemistry Class 6 Textbook Solutions

Selina Concise Chemistry Class 6 ICSE Solutions – Air and Atmosphere

ICSE SolutionsSelina ICSE SolutionsML Aggarwal Solutions

APlusTopper.com provides step by step solutions for Selina Concise ICSE Solutions for Class 6 Chemistry. You can download the Selina Concise Chemistry ICSE Solutions for Class 6 with Free PDF download option. Selina Publishers Concise Chemistry for Class 6 ICSE Solutions all questions are solved and explained by expert teachers as per ICSE board guidelines.

Selina Class 6 Chemistry ICSE SolutionsPhysicsBiologyMathsGeographyHistory & Civics

Selina Concise ICSE Solutions for Class 6 Chemistry Chapter 6 Air and Atmosphere

POINTS TO REMEMBER

1. Air cannot be seen as it is transparent we only feel its presence when it blows.
2. The thick layer of air around earth upto 320 km above the surface is called atmosphere.
3. Air is a mixture of various gases, water vapours, which protects us from harmful rays of Sun. It consists of nearly 80% nitrogen, 20% oxygen.
4. The components of air can be separated by simple physical means. Has no formula.
5. N₂ is inert gas, reduces the effect of O₂ present in air, N₂ is helpful in plant growth as it provides nitrates used by plant for formation of protein which is important nutrient for animals.
6. Oxygen is supporter of combustion no burning is possible without O₂ and no life is possible without it.
7. CO₂ is must for plant life, plants prepare their food taking CO₂ from air. CO₂ maintains temp, of earth and without CO₂ our planet would be too cold.
8. Ozone in the upper layers of atmosphere protects the earth from harmful ultraviolet rays of Sun.
9. Air is useful in many ways : Acts as medium for propagation of sound, birds, insects to fly in air, air ships, aeroplanes to fly only must due to presence of air. Air helps in movement of yachts, parachutes, aircrafts etc. Air is must for animals and plants on earth and air dissolved in water for aquatic life.
10. Respiration : Is a slow chemical process when O₂ present in the inhaled air reacts with the digested food material in the body to release energy, CO₂ and water. Sugar + Oxygen -> Carbon dioxide + Water + Energy.
    Breathing : The complete process in which air is inhaled and exhaled is called breathing.
11. Combustion : Burning of substance in air completely producing CO₂, heat and light.
12. Photosynthesis : Preparation of food by green plants taking CO₂ from air, water from soil in presence of sun light.
13. Oxygen in air remains constant it is not depleted as plants during day time produce lot of O₂ in air.
14. Soil contains air trapped in between its particle and organisms living there breath this oxygen. Due to this earth worms come out of soil during heavy rains as water gets into the pores in the soil.
15. Common pollutant of air are smoke, dust, coal, emitted by industries and automobiles.
16. Gases like SO₂, NO₂ also pollute air as they form corresponding acids when they come in contact with water vapours in air and fall down in the form of acid rain with rain water.
17. These pollutants reduce the visibility which lead to accidents, cause global warming.

EXERCISE-I

Question 1.
What is atmosphere ?
Answer:
The earth is surrounded by a thick layer of air called the atmosphere that extends upto a height of about 320 kilometres above the surface of the earth.

Question 2.
Why can’t we see air ?
Answer:
We can’t see air because it is colourless, odourless and transparent gaseous matter.

Question 3.
What is wind ?
Answer:
Fast moving air is called wind.

Question 4.
What would have happened if there would have been no atmosphere around the earth ?
Answer:
Without atmosphere life would not be possible as atmosphere protects us from harmful gases. We could not live without
air present in atmosphere. In absence of the atmopshere, the earth would get so cold at night that we would not be able to survive. No CO₂ and N₂ for plants without atmosphere.
P.Q. When water is heated, we see bubbles rising up. Why ?
Answer:
Water has a lot of air dissolved on it. This is what allows breathing to fishes and other aquatic beings. The solubility of gases decreases when the temperature is raised, and that is why the dissolved air bubbles out from the water.

Question 5.
Why is air called a mixture ? Give five facts in support of your answer.
Answer:
Air is a mixture because :

1. Air has no formula, a mixture has no formula whereas compound has a formula.
2. No energy changes are involved to form air from various gases.
3. When air is formed out of its constituents no change in mass and no change in volume takes place.
4. Properties of air vary from place to place and time to time. i.e. there is more CO₂ in towns as compared to villages where more oxygen prevails as compared to towns.
5. Components of air can be separated by simple physical methods.

Question 6.
What are the main components of air ? Write down the composition of three main gases present in air by volume.
Answer:
Main components of air are :

Question 7.
What do you observe when

(a) Ice cold water is filled in a glass tumbler.
(b) A burning candle is covered with an inverted jar.
(c) Carbon dioxide gas is passed through lime water.
(d) A beam of light is allowed to enter in a closed dark room through a small hole.

Answer:

(a) We will observe that fine water droplets get deposited on the outer wall of the glass tumbler.
(b) The candle burn more brightly because candle gets oxygen support in burning.
(c) When CO₂ gas is passed through lime water it forms calcium carbonate which is white precipitate (turbidity). This gives the milky white appearance to the solution.
(d) We will observe randomly moving dust particles in the beam of light. This confirms the presence of dust particles in the air.

Question 8.
Write the chemical name of –

(a) Lime water
(b) The white insoluble solid formed on reaction of carbon dioxide with lime water.

Answer:

(a) Calcium hydroxide Ca(OH)₂
(b) Calcium carbonate – CaCO₃

These droplets were present in air as moisture which got condensed on the cooler walls of glass tumbler. This shows the presence of water vapours in air.

EXERCISE-II

Question 1.
Name two important processes supported by oxygen present in air.
Answer:
Two processes supported by oxygen present in air are :

1. Combustion
2. Respiration

Question 2.
Give two uses of the following components present in air:

(a) oxygen
(b) nitrogen
(c) carbon dioxide
(d) water vapour

Answer:

(a) Oxygen: For respiration and breathing no life is possible without oxygen.
Oxygen is the most vital component which is responsible for two most important processes,
(i) Respiration and
(ii) Combustion.
(b) Nitrogen : Present in air gets fixed up in the soil as nitrates used by plants for their growth and for the formation of protein an important nutrient for animals. It is used to make fertilizers and various nitrogenous products.
(c) Carbon dioxide : Carbon dioxide is used by plants to prepare their food by photosynthesis. Dry ice (solid carbon-dioxide) is used as a refrigerant. Carbon dioxide is used to prepare fizzy drinks like soda water.
(d) Water vapour : Water vapors present in air provide moisture for both plants and animals. It also helps in predicting climatic conditions of a particular area as its amount varies from place to place and time to time.

Question 3.
Define the following:

(a) Respiration
(b) Photosynthesis
(c) Combustion.

Answer:

(a) Respiration : Respiration is a chemical process that takes place in all living beings (slow in plants). In this process, oxygen present in inhaled air reacts with the digested food material in the body. This results in release of energy, carbon-dioxide and water.
Sugar + Oxygen → CO₂ + Water + Energy.
(b) Photosynthesis : Process of producing food by green plants taking CO₂ from air, water from soil by roots and in presence of sun light and chlorophyll is called photosynthesis.
(c) Combustion : Combustion also called burning “is burning of substance in oxygen of air completely producing heat and light is called combustion.”

Question 4.
What are fuels ? Give two examples of modern fuels.
Answer:
Fuels : The substances which burn in air to produce large amount of energy in the form of heat and light are called fuels.
Examples :

1. CNG (Compressed natural gas)
2. LPG (Liquefied natural gas)

Question 5.
Give reasons :

(a) Aquatic animals and plants are able to survive in matter.
(b) A burning candle stops burning if covered with a glass tumbler.
(c) Mountaineers and divers carry oxygen cylinders with them.
(d) When water is heated, we see bubbles rising up.

Answer:

(a) Aquatic animals and plants use oxygen dissolved in water and survive.
(b) Oxygen is necessary for combustion when covered with a glass tumbler supply of oxygen stops.
(c) For artificial respiration mountaineers carry oxygen cylinders as at high altitudes, the air is thin and breathing becomes difficult. Also divers carry oxygen cylinder for artificial respiration as there is less oxygen dissolved in water (less dense) and breathing becomes difficult.
(d) These bubbles come from the air dissolved in water. The marine life uses air dissolved in water.

Question 6.
Name the processes which maintain the balance between oxygen and carbon dioxide in the air. How is it done?
Answer:
Respiration and combustion are the processes which maintain the balance between oxygen and CO₂ in the air.

1. Respiration : Respiration is a chemical process that takes place in all living beings. In this process, oxygen present in the inhaled air reacts with the digested food material in the body. This results in the release of energy, carbon dioxide and water.
2. Combustion : Burning or combustion is a process in which a substance reacts chemically with oxygen and gets oxidised, with the release of energy in the form of heat and light. It is a fast process. During the process of burning, along with energy, carbon dioxide and water vapour are also produced.

Question 7.
State two similarities and two differences between respiration and burning.
Answer:
Similarities:
Burning

1. Oxygen is needed to combine with carbon and hydrogen in compound.
2. CO₂ and H₂O are formed with release of energy.

Respiration

1. Oxygen is needed tocombine with C and H2 of food.
2. CO₂ and H₂O are formed with release of energy.

Differences :
Burning

1. It occurs at higher temperature.
2. Is fast process.
3. A natural and continous process.

Respiration

1. It occurs at body temperature.
2. Is slow process.
3. An artificial & discont inuous process.

Question 8.
Define rusting ? What are the two necessary conditions for rusting of iron. Give the chemical name of rust.
Answer:
Rusting : Slow conversion of iron into its hydrated oxide in the presence of moisture and air is called rusting.
Conditions for rusting:

1. Presence of moisture (water).
2. Presence of oxygen (air).

Chemical name of rust is hydrated iron oxide [Fe20rxH20]

Question 9.
How is air useful to :

(a) water boats
(b) agriculture
(c) windmills
(d) scooters and cars.

Answer:

(a) Air helps movement of water boats.
(b) Air speeds up drying up of agricultural products like grains, pulses fruits etc. Air helps in pollination of flowers and dispersel of seeds.
(c) Windmills work where there is sufficient movement of air.
(d) Air filled tyres of cars move smoothly on road as there is less friction.

Question 10.
State the full form of LPG and CNG ? How are the two different in their composition ?
Answer:
LPG (Liquefied Petroleum Gas): It is obtained from crude petroleum oil. It mainly contains gaseous compounds known as isobutane and butane. Popularly it is known as cooking gas. It is the best fuel for domestic purposes and in laboratories. It is available in cylinders. It is also supplied through pipes in big cities.
CNG (Compressed Natural Gas): It is produced along with crude oil. It mainly contains methane gas. It has become a popular fuel for vehicles like three wheeler scooters, cars and buses. It is a cheap fuel as well as pollution free. It is used as a substitute of petrol.
Difference in composition
LPG is obtained from crude petroleum oil. It mainly contains gaseous compounds known as isobutane and butane. While
CNG is produced along with crude oil. It mainly contains methane gas.

Question 11.

(a) Why is nitrogen important to all living beings ?
(b) What is nitrogen fixation ?

Answer:

(a) Nitrogen constitutes 78% of air by volume. It is of vital importance to the plants, animals and human beings as it is needed to prepare vital nutrient ‘protein’ to every living being which is necessary for their growth.
(b) Nitrogen cannot be absorbed directly by plants. It is first fixed up in the soil as nitrites and nitrates and then absorbed by the plants in soluble forms. This phenomenon is called nitrogen fixation.

EXERCISE-III

Question 1.
What is air pollution ?
Answer:
Air Pollution : “Mixing of UNWANTED and HARMFUL SUBSTANCES in air is called AIR POLLUTION.”

Question 2.
Mention five causes of air pollution.
Answer:
CAUSES OF AIR POLLUTION ARE:

1. Burning of FOSSIL FUELS and FIBRES.
2. Cutting of forests.
3. Erruption of volcanoes.
4. Increase in POPULATION.
5. Agricultural activities like use of chemical fertilisers, insecticides, pesticides and burning of husks etc.

Question 3.
Name two air pollutants which

(a) affect our health
(b) cause acid rain
(c) cause global warming.

Answer:

(a) Two air pollutants that affect our health are :

1. DUST
2. SMOKE

(b) Two air pollutants that cause acid rain are :

1. SULPHUR DIOXIDE (SO₂) and
2. NITROGEN DIOXIDE (NO₂)

(c) Two air pollutants that cause global warming are :

1. CARBON DIOXIDE (CO₂)
2. METHANE (CH₄)

Question 4.
What is meant by ozone depletion?
Answer:
Ozone is present in the upper layer of atomosphere called stratosphere. It protects the earth from harmful ultraviolet rays of the sun.
Some air pollutants like chlorofluorocarbon (CFC) react with ozone present in the atmosphere. These pollutants reduce the density of the air. The ultra-violet rays from the sun reach directly on the earth which affect human health, causing skin diseases and cancer and also increase the earth’s temperature.

Question 5.
State four steps to be taken to control air pollution.
Answer:
Four steps taken to control air pollution :

1. Tall chimneys be installed in factories and power houses so that smoke and gases rise high and get diluted.
2. GROW more tall trees to absorb CO₂ Also plants help in bringing rains.
3. Use unleaded petrol and CNG in all public transport vehicles
4. Industries should be located far away from residential areas.

Question 6.
Name three greenhouse gases.
Answer:
Carbon dioxide, methane, nitrous oxide, etc.

OBJECTIVE TYPE QUESTIONS

1. Fill in the blanks

(a) The layer of air around the earth is called the atmosphere.
(b) Although we cannot see air, we can feel it.
(c) Air is a mixture of gases.
(d) Plants and animals maintain the balance of carbon- dioxide and oxygen in air.
(e) Polluted air is harmful for health.
(f) The supporter of combustion in air is oxygen.
(g) Green plants need CO₂ water and light to prepare food.
(h) Oxygen is used in burning the food to get energy.
(i) Aquatic plants and animals use dissolved oxygen in water.
(j) The reddish brown powder on the surface of iron nails exposed to air and moisture is called rust.

2. Write ‘true’ or ‘false for the following

(a) Air is a compound.
Answer. False
Correct: Air is a mixture.

(b) Carbon dioxide is given out during photosynthesis.
Answer. False
Correct : Oxygen is given out during photosynthesis.

(c) Respiration needs nitrogen.
Answer. False
Correct: Respiration needs oxygen.

(d) The composition of air was discovered by Lavoisier.
Answer. True

(e) The major component of air is oxygen.
Answer. False
Correct: The major component of air is nitrogen.

MULTIPLE CHOICE QUESTIONS

Tick (√) the correct alternative from the choice given for the following statements

1. Air consists of

1. only oxygen
2. only nitrogen
3. only carbon dioxide
4. all of these

2. Air pollution is due to the

1. cutting of green plants
2. gases like carbon monoxide, sulphur dioxide etc.
3. smoke given out by factories
4. all of the above

3. The gases which cause acid rain are

1. sulphur dioxide and oxygen
2. nitrogen and oxygen
3. carbon dioxide and water vapour
4. nitrogen dioxide and sulphur dioxide

4. Rust is

1. hydrated iron oxide
2. hydrated copper sulphate
3. anhydrous iron oxide
4. none of the above

5. Photosynthesis is a process in which plants

1. take in oxygen and give out carbon dioxide
2. take in carbondioxide and give out oxygen
3. take in nitrogen and give out oxygen .
4. none of the above.

6. Fuels which do not leave any residue on burning are

1. coal and wood
2. coal and LPG
3. wood and CNG
4. LPG and CNG

7. Respiration

1. is a slow process
2. is a natural and continuous process
3. takes place at body temperature
4. all of the above

8. Which of the following is common in combustion and respiration

1. oxygen
2. release of heat and light
3. natural process
4. nitrogen

9. Which of the following is not a green house gas ?

1. carbondioxide
2. sulphur dioxide
3. methane
4. nitrogen

10. The substance which accelerates the speed of a reaction without itself undergoing any change is called

1. catalyst
2. pollutant
3. fuel
4. none of the above.

Selina Concise Chemistry Class 6 ICSE Solutions – Elements, Compounds, Symbols and Formulae

ICSE SolutionsSelina ICSE SolutionsML Aggarwal Solutions

APlusTopper.com provides step by step solutions for Selina Concise ICSE Solutions for Class 6 Chemistry. You can download the Selina Concise Chemistry ICSE Solutions for Class 6 with Free PDF download option. Selina Publishers Concise Chemistry for Class 6 ICSE Solutions all questions are solved and explained by expert teachers as per ICSE board guidelines.

Selina Class 6 Chemistry ICSE SolutionsPhysicsBiologyMathsGeographyHistory & Civics

Selina Concise ICSE Solutions for Class 6 Chemistry Chapter 4 Elements, Compounds, Symbols and Formulae

POINTS TO REMEMBER

1. Pure substances : “A single substance of definite composition.” Pure substances are homogeneous. They are made up of only one kind of atoms and compounds or made up of only one kind of molecules.
2. Elements : An element is defined as a pure substance made up of only one kind of atoms that cannot be converted into anything simpler than itself by any physical or chemical process.
3. Metals : Most of the elements known to us are metals. Example: Sodium potassium, iron, gold, silver are elements. Metals are lustrous, hard, ductile and malleable, good conductor of electricity. Mercury (liquid) have High M.P. and High B.P. but exceptions are there.
4. Non-metals : Carbon, phosphorous, sulphur all solids, Bromine (liquid) iodine(s) and rest are gases, dull, do not shine. Cannot be drawn into wires, non malleable, have low M.P. and B.P. do not produce sound when struck, do not conduct electricity. Exceptions are there.
5. Boron, sillicon, arsenic, antimony resemble both metals and non-metals and are called metalloids.
6. Unreactive gases, also called noble gases are gases which are very unreactive i.e. He, Ne Ar, Kr, xenon and radon.
7. Symbols are abbreviations that are used to denote a chemical element which is usually first letter of its name in English or Latin.
8. O is the symbol of element oxygen.
   C is symbol of element carbon.
   Cu is symbol of element copper (taken from Latin name Cuprum)
9. Compound is formed by the chemical combination of two or more elements in definite ratio (by mass).
10. Molecule is the smallest unit of a compound.
11. Elements are the basic substances from which all other substances are made.
12. Compound : Consist of two or more elements combined in definite proportion, pure and homogeneous, physical and chemical properties are entirely new and different from its consititutent elements, energy is either needed or produced when a compound is formed.
13. Atoms : Atom is the smallest unit of an element which cannot be further broken into simpler parts, may or may not have independent existence.
14. Molecule of element : When two or more atoms of the same element combine it forms a molecule of an atom. e.g. N₂,O₂
15. Molecule of compounds : When atoms of two or more element combine, they form a molecule of a compound. H₂0, HC1, CuSO₄.

EXERCISE-I

Question 1.
Classify the following substances into elements and compounds.
Answer:
Mercury, sulphur, sugar, water, sand, gold, coal, oxygen, alcohol.
Ans. Element: Mercury, sulphur, gold, coal, oxygen.
Compound : Sugar, water, sand, alcohol.

Question 2.
Give the symbols of: Carbon, calcium, copper, chlorine, cobalt, argon.
Answer:
Carbon is C         Chlorine is Cl
Calcium is Ca     Cobalt is Co
Copper is Cu       Argon is Ar

Question 3.
Define a pure substance. How many types of pure substances do you know ?
Answer:
Pure substances : “A substance of a definite composition which has consistent properties throughout, is called a pure substance”
Types of pure substances: Pure substances are of two types (i) Elements, (ii) Compounds.

Question 4.
Define : (a) Elements (b) Compounds.
Name the particles from which elements and compounds are made of.
Answer:
(a) Elements : An element is defined as a pure substance made up of only one kind of atoms that cannot be converted into anything simpler than itself by any physical or chemical process.
(b) Compounds : Compounds are pure substances composed of two or more elements in definite proportion by mass and has a definite set of properties. Compound is made up of only one kind of molecules.

Question 5.
Give two examples for each of the following :
(a) Metals
(b) Non-metals
(c) Metalloids
(d) Noble gases
Answer:
(a) Metals : Iron, silver, gold.
(b) Non-metals : Carbon, sulphur, oxygen.
(c) Metalloids : Antimony, silicon, boron.
(d) Noble gases : Helium, argon, neon.

Question 6.
Name the elements which form water. How will you justify that water is a compound ?
Answer:
The elements which form water are (i) Hydrogen and oxygen.
Justification : Water has entirely different properties (i.e. is a liquid, extinguishes fire) from the elements it is made up of i.e. Hydrogen a gas catches fire oxygen a gas supporter of combustion.

1. Energy is needed to form water on combining O₂ with H₂.
2. We can not seperate the constituents of water by simple physical means.

Question 7.
Give three differences between metals and non-metals.
Answer:
Metals

1. Metals are ductile i.e. can be drawn into wires.
2. Metals are malleable i.e. can be beaten to form sheets.
3. They are sonorous.

Non-metals

1. Non-metals are mostly soft solids cannot be drawn into wires.
2. They are mostly gases and are not malleable.
3. They donot produce sound when struck.

Question 8.
How is sodium chloride different from its constituent elements, sodium and chlorine ?
Answer:
Sodium is a metal that is stored in kerosene oil as it reacts very fast with air and water. Chlorine is a reactive greenish yellow gas which is poisonous. When these two elements combine chemically they form common salt sodium chloride which is non poisonous colourless solid substance that we use in our food to add taste and to obtain some nutrition.

Question 9.
State four important characteristics of compounds.
Answer:

1. When compound is formed energy like heat, light or electricity is either needed or produced.
2. A compound has properties entirely different from the properties of its constituents.
3. Change in weight takes place.
4. It cannot be separated into its constituents by simple physical means.

Question 10.
Give two examples for each of the following :

(a) Non-metals which are solids
(b) Metals which are soft
(c) Non-metals which are lustrous
(d) Elements which are liquids.
(e) Inert gases
(f) Metalloids

Answer:

(a) Phosphorus, Sulphur
(b) Lead and Sodium
(c) Radium, Graphite
(d) Mercury, Bromine
(e) Helium, Neon
(f) Antimony, Arsenic

Question 11.
Name the elements present

(a) Sugar
(b) Ammonia
(c) Marble
(d) Washing soda

Answer:

Compounds
(a) Sugar
(b) Ammonia
(c) Marble
(d) Washing soda
Elements present
(a) Carbon, hydrogen & oxygen
(b) Nitrogen and hydrogen
(c) Calcium, carbon & oxygen
(d) Sodium, carbon & oxygen

Question 12.
What is the proportion of elements present in the following compounds?
(a) H₂O
(b) CO₂
(c) CaO
(d) NO₂
Answer:

Question 13.
Name two compounds which dissolve in water.
Answer:
Two compounds which dissolve in water are sugar, table salt.

EXERCISE-II

(ATOMS & MOLECULES)

Question 1.
Define:

(a) Atom
(b) Molecule
(c) Atomicity
(d) Formula

Answer:
(a) Atom : An atom is the smallest indivisible unit of an element which exhibits all the properties of that element and may or may not have independent existence.
(b) Molecule : A molecule can be defined as the smallest unit of an element or a compound which exhibits all the properties of that element or compound and has independent existence. They are divisible into atoms.
(c) Atomicity : The number of atoms in a molecule of an element is called its atomicity.
(d) Formula : Formula is a short way of representing the molecule of an element or a compound.

Question 2.
Why are symbols and formulae of substances important?
Answer:
Importance of symbols and Formulae :
Symbols and formulae of substance gives a lot of information like.

1. Types of elements present in the compound. E.g. (H₂0 is made of two elements hydrogen and oxygen).
2. Number of each kind of atoms in one molecule. E.g. (water has 2 atoms of hydrogen combined with 1 atom of oxygen.)
3. Mass of one molecule of the compound. E.g. [H₂O has mass (1 × 2) + 16 = 18 g].

Question 3.
Mention three gaseous elements and write their molecular formulae.
Answer:

Question 4.
State the informations obtained from the formula of a compound.
Answer:
A formula gives us the following information about a compound.

1. Types of elements present in the compound.
2. Number of each kind of atoms in one molecule of the compound.
3. Mass of one molecule of the compound.

Example:
A molecule of carbon dioxide gas is represented by CO₂ It indicates that a carbon dioxide molecule is formed by the combination of two elements i.e. carbon and oxygen. The number of carbon atom is one and that of oxygen atom is two. The mass of one molecule of carbon dioxide can be calculated by adding the mass of one atom of carbon and two atoms of oxygen.

Question 5.
What is meant by
(a) 2H and H₂
(b) H₂0 and 3H₂O ?
Answer:
(a) 2H is two atoms of hydrogen. H₂ is one molecule of hydrogen gas.
(b) H₂0 represents one molecule of water. 3H₂0 represents 3 molecules of water.

Question 6.
State the number of atoms of each kind, present in
(a) C₆ H₁₂O₆
(b) H₂SO₄
(c) HNO₃
(d) CaCO₃
Also name these compounds.
Answer:

Question 7.
Write the molecular formulae of compounds calcium oxide, hydrogen sulphide, carbon monoxide and lead sulphide.
Answer:
Compound Calcium oxide is formed of elements calcium (Ca) and oxygen (O)

Symbols combining power Here subscript number is same Ca₂ Formula of calcium oxide is CaO Compounds Hydrogen sulphide is formed of elements

EXERCISE-III

Question 1.
Name:
(a) Three different forms of carbon.
(b) A form of carbon used as a gem.
(c) Two substances used to make electric wires.
(d) Two substances used to make jewellery.
(e) A substances used as an insulator.
Answer:
(a)

1. Diamond
2. Graphite
3. Coal

(b) Diamond is used as gem.
(c)

1. Copper.
2. Aluminium as these are good conductors of electricity.

(d)

1. Gold.
2. Silver as these are shining, lustrous, and ductile.

(e) Plastic is used as insulator as it is bad conductor of electricity.

Question 2.
Give one use of each of the following substances :

(a) Iron
(b) Brass
(c) Coal

Answer:
(a) Iron : To make machines tools and building material.
(b) Brass : To make water taps and utensils.
(c) Coal: Coal is used as fuel also used in thermal power plant to produce electricity.

Question 3.
Give reason:

(a) A frying pan is made up of steel but its handle is made up of wood.
(b) Graphite is used to make lead of the pencils.
(c) Argon is filled in electric bulbs.

Answer:

(a) Steel is good conductor of heat to cook food, pan is made of steel where as wood is insulator of heat and to hold, handle is made up of wood.
(b) Graphite leaves mark on the paper and makes it black.
(c) Argon is inert gas and protects the element of bulb from oxidation and burning. Hence increases bulb’s life.

Question 4.
Answer the following questions:

(a) Why are copper and aluminium used to make electric wires?
(b) What do you understand by the statement: ‘metals are ductile and malleable’?
(e) Give the advantages of using symbols instead of names of elements or compounds.
(d) When iron is mixed with sulphur at room temperature, it does not form a compound. Why?
(e) Find the atomicity of the Tollowing molecules:

1. calcium chloride
2. aluminium suiphide
3. acetic acid
4. dinitrogen oxide
5. carbon monoxide

Answer:
(a) Copper and aluminium are good conductors of heat and electricity. They can be drawn into wires and beaten into sheets. Therefore, they are used to make electric wires.
(b) Metals are ductile, i.e., they can be drawn or stretched into thin wires. They are malleable, i.e., they can be beaten into thin sheets.
(c)

1. Symbols increases scientific communication across the world.
2. Symbols helps to make equations and data shorter and concise.
3. Symbols are helpful for Scientists as it would take time and paper to do their job. They had to write out the full name of every element instead of its symbol.

(d) When iron is mixed with sulphur at room temperature, it does not form a compound because the mixture of iron and sulphur requires heat to form a compound i.e. iron sulphide.

OBJECTIVE TYPE QUESTIONS

1. Fill in the blanks

(a) Atomicity refers to the number of atoms in the molecule of an element.
(b) The most abundant element in the earth’s crust is oxygen.
(c) A metal which is a liquid at room temperature is mercury.
(d) The most abundant element in the atmosphere is nitrogen.
(e) A metal which is a poor conductor of electricity is tungsten.
(f) A diatomic gaseous element is oxygen.
(g) A liquid non-metal is bromine.

2. Match the columns

3. Indicate whether the following statements are true or false.

(a) A compound is made up of just one kind of atom.
Answer. True False
Correct : A compound is made up of two or more elements is a fixed proportion by mass.

(b) Metals reflect light and are good conductors of electricity.
Answer. True

(c) Metals can be polished.
Answer. True

(d) Elements are made up of compounds.
Answer. False
Correct : Elements are made up of atoms.

(e) All elements are artificially prepared.
Answer. False
Correct: All elements are made up of a limited number of basic substances.

(f) Molecules can exist independently.
Answer. True

(g) Molecules combine to form atoms.
Answer. False
Correct : Atoms combine to form molecule.

(h) Noble gases are high reactive.
Answer. False
Correct: Noble gases are non-reactive.

(i) Ozone is a triatomic molecule.
Answer. True

MULTIPLE CHOICE QUESTIONS

Tick (√) the correct alternative from the choice given for the following statements :

1. All pure substances have

1. the same physical state .
2. the same colour
3. the same composition
4. a definite set of properties

2. Sugar is a compound which consists of the elements

1. carbon and hydrogen
2. hydrogen and oxygen
3. carbon, hydrogen and oxygen
4. hydrogen, carbon and sulphur

3. Atoms of different kinds combine to form molecules of

1. an element
2. a compound
3. a mixture
4. all of the above

4. Sulphur and carbon are

1. metals
2. non-metals
3. metalloids
4. noble gases

5. Gold is used to make jewellery because

1. it is dull
2. lustrous and attractive
3. highly reactive
4. very cheap

6. The most abundant elements in the universe are

1. neon and argon
2. hydrogen and helium
3. aluminium and copper
4. oxygen and nitrogen

7. The compound used as common salt is

1. sodium chloride
2. calcium chloride
3. sodium oxide
4. hydrogen chloride

8. Brass and bronze are

1. elements
2. mixtures
3. compounds
4. all of the above

9. Sand is a compound of

1. silicon and nitrogen
2. silicon and oxygen
3. oxygen and sulphur
4. none of the above

10. From the list given below select the correct substance which is most suitable to the statements given : (oxygen, diamond, zinc, graphite, gold)

1. A metal which is brittle.
2. A non-metal which is a good conductor of electricity.
3. The hardest naturally occurring substance.
4. The most ductile metal.
5. A gaseous non-metal.
   Answer.
   (a) Zinc
   (b) Graphite
   (c) Diamond
   (e) Oxygen

Selina Concise Chemistry Class 6 ICSE Solutions – Water

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APlusTopper.com provides step by step solutions for Selina Concise ICSE Solutions for Class 6 Chemistry. You can download the Selina Concise Chemistry ICSE Solutions for Class 6 with Free PDF download option. Selina Publishers Concise Chemistry for Class 6 ICSE Solutions all questions are solved and explained by expert teachers as per ICSE board guidelines.

Selina Class 6 Chemistry ICSE SolutionsPhysicsBiologyMathsGeographyHistory & Civics

Selina Concise ICSE Solutions for Class 6 Chemistry Chapter  7  Water

POINTS TO REMEMBER

1. Water cannot be replanished and without water we cannot live.
2. 4 / 5 th of earth’s surface is covered with water and 1% of this available water is in the form of rivers, lakes, ponds and under ground water. It is utilized for agriculture and living beings consumption.
3. In the atmosphere water is present in the form of vapour, mist, clouds etc.
   
4. Distillation : The process of removal of the dissolved salts from sea/ocean water.
5. Purest form of water is rain water as it is formed by evaporation and condensation.
6. Sea water is unfit for agriculture as plants do not tolerate saline water.
7. Change of water from water bodies (lakes, rivers and oceans) into vapours in air and condensing of vapours and fall in the form of snow, water is called water cycle and water cycle plays an important role in
   (i) restoring the lost water from earth’s surface
   (ii) in controlling the climatic conditions.
8. Water vapour is one of the invisible gases which makes up the air we breathe.
9. Drinking water may contain some minerals and salts but it must be free from suspended impurities, harmful bacteria and germs.
10. Sometimes water from a source look clear but may contain germs which cause water borne diseases like typhoid, cholera, jaundice, dysentry, gastroenteritis etc.
11. Water can be made fit for drinking by
    (i) boiling,
    (ii) sterilisation
    (iii) exposing to air and sunlight
    (iv) by chemical treatment like chlorination or ozonisation.
12. Water is universal solvent i.e. water can dissolve nearly in every substance.
13. Distilled water is the purest form of water but not good for drinking purposes. It is good for medicinal purposes, laboratories and batteries.
14. Both stirring and heating help in dissolving the substance in water.
15. “The amount of solute that dissolves in a given quantity of solvent at a given temperature is called solubility of the solute.”
16. Air dissolved in water helps aquatic animals and plants to survive.
17. Eutrophication : “The deficiency of oxygen in the water may cause death of aquatic animals. This loss of dissolved oxygen from water in water bodies is called eutrophication.”

EXERCISE-I

Question 1.
Name:

(a) Three major sources of natural water
(b) Four sources of surface water.
(c) Two underground sources of water.

Answer:

(a) Three major sources of natural water :

1. Surface water (sea water)
2. Above surface water (rain)
3. Underground water (springs)

(b) Four sources of surface water sources :

1. Ocean water
2. River water
3. Lake water
4. Glaciers

(c) Two sources of underground water :

1. Well water
2. Spring water

Question 2.
Answer the following questions in short:

(a) In which form is water present in the atmosphere ?
(b) Which source of water contains the highest concentration of salt in it ?
(c) Why is rain water considered the purest form of natural water?
(d) What possible impurities does rain water contain ?
(e) What is water table ?
(f) Why is spring water pure enough for drinking but unsuitable for laboratory use ?
(g) Why the taste of spring water differs at different places ?

Answer:

(a) Water is present in the form of water vapours in the atmosphere.
(b) Sea water contains highest concentration of salt.
(c) Rain water is the purest form of water as it is distilled water i.e. water from surface of earth has evaporated and then vapours condense in the atmosphere.
(d) The impurities present in rain water are dust and dissolved gases like oxygen, nitrogen and carbon dioxide and these gases are not poisonous, rain water is safe for drinking.
(e) Level of ground water is called water table.
(f) Spring water is free from suspended impurities and germs (harmful bacteria) as water has been filtered through different layers of soil and is fit for drinking. Spring water contains dissolved impurities which are due to the nature of soil surrounding the spring and cannot be used for laboratory purposes, where distilled water free from impurities is needed.
(g) Spring water contains unsolved impurities which have entered the water from surrounding soil and soil impurities (salts) differ from place to place. Hence taste of water differs from place to place.

Question 3.
List three major impurities present in river water.
Answer:
Three major impurities present in river water :

1. Suspended impurities like clay and sand particles.
2. Harmful bacteria.
3. Mineral salts.

Question 4.
Give the percentage of water in the following :

(a) Rice and wheat grains
(b) Eggs
(c) Tomatoes
(d) Bread
(e) Water melon

Answer:
Percentage of water in :

(a) Rice — 3 % to 4% Wheat grain — 3% to 4%
(b) Eggs —75%
(c) Tomatoes — 95%
(d) Bread —25%
(e) Water melon — 97%

Question 5.
What are the three states of water ?
Answer:
Three states of water are :

1. Ice (solid)
2. Liquid (water)
3. Gaseous (steam)

Question 6.
Why are ice, liquid water and steam considered to have the same chemical substance ?
Answer:
Ice, liquid water and steam have same composition i.e. 2 parts hydrogen and one part oxygen by volume and same formula H₂O, can be easily transformed from one state to other by heating or cooling.

Question 7.
How is a cloud formed ?
Answer:
Water from the surface of earth, lakes, seas, rivers, formed by plants (transpiration) etc. evaporates and rises high up in the atmosphere and condense into water droplets and collect to form cloud.

Question 8.
What is water cycle ? What is its importance ?
Answer:
Water cycle : The change of water from one form to another in nature which results in continuous circulation of water from earth’s surface to the atmosphere and from the atmosphere back to the earth’s surface is called water cycle.
Importance of water cycle:

1. It assures a continuous supply of water to us.
2. In controlling the climatic conditions all over the world.

Question 9.
How are the following formed ?

(a) fog
(b) mist
(c) dew
(d) frost

Answer:

(a) Fog : When water vapours change into tiny droplets of water near the ground, fog is formed.
(b) Mist: When tiny droplets of water remain suspended in air it is called mist.
(c) Dew : When the water vapour condenses on cold objects like grass, leaves and flowers in winter in the form of tiny droplets of water called dew.
(d) Frost: When the dew freezes it is called frost.

EXERCISE-II

Question 1.
Name:

(a) Two chemicals used to destroy germs present in water.
(b) Two diseases which spread through impure water.
(C) A chemical used for loading.
(d) Two substances which add taste to water.
(e) Two household methods to get safe drinking water.

Answer:
(a) Two chemicals used to kill germs in water are:

1. Chlorine.
2. Potassium permanganate.
3. terilizing water with ozone.

(b) Two diseases are cholera, dysentry.
(c) Chemical used for loading is potash alum.
(d) Minerals and carbon dioxide gas adds taste to water.
(e) Methods to get safe drinking water:

1. Adding chlorine tablets.
2. By adding potassium permanganate crystals.

Question 2.
Answer in brief:

(a) Why is river water unfit for drinking?
(b) Why is tap water a mixture?
(c) What is mineral water?
(d) What is the purpose of adding bleaching powder to water supplied to the town?
(e) How is chemically pure water obtained in the laboratory?
(f) how is water in swimming pool kept free a germs?

Answer:

(a) River water contains mineral salts, suspended impurities like clay, sand particles, organic matter and bacteria and is not fit for drinking.
(b) Tap water contains, minerals, air, chlorine and other dissolved impurities that varies from place to place, therefore it is a mixture.
(c) Mineral water is pure water fit for drinking. It is collected from natural source and contains air, minerals and salts free from suspended impuiities, harmful bacteria and germs.
(d) Adding bleaching powder to water, kills germs and harmful bacteria and viruses present in water.
(e) Chemically pure water for laboratories is obtained-by distillation. Distilled water does not contain any salt or mineral
(f) Water in swimming pool kept free from infections and germs by chlorination i.e. treating water with chlorine gas.

Question 3.
Define:

(a) Sterilisation
(b) Sedimentation
(c) Loading
(d) Aeration

Answer:

(a) Sterilisation : The process of removal of microorganisms including bacterial spores from water to avoid water borne diseases is known as sterilisation.
(b) Sedimentation : The setting of suspended solid matter at the bottom of a liquid is called sedimentation.
(c) Loading : The process of adding a chemical to an impure liquid in order to increase the speed of sedimentation of suspended particles is called loading.
(d) Aeration : To kill harmful micro-organisms present in filtered water, air underpressure is blown into the filtered water. This process is called aeration.

Question 4.
What is potable water ? List four characteristics of potable water.
Answer:
Potable water : Water fit for drinking purposes is called potable water.
Four characters of drinking water :

1. It should be transparent.
2. Should have no colour, no odour.
3. Should be free from harmful bacteria and germs.
4. It should contain same salts and minerals needed by the body CO₂ to add to taste.

Question 5.
Why is water important for plants and animals ?
Answer:
Importance of water for plants:

1. Plants need water to prepare their food, for germination and growth to produce fruits, flowers etc.
2. For conduction of food prepared by plants to other-parts of plant i.e., for translocation.
3. Large number of plants live in water; water provides nutrients and oxygen for their survival.
   Crops need water for their growth. Water in the form of rain, washes the dust and smoke deposited on leaves by vehicles and helps the stomata in exchanging gases.

Importance of water for animals:

1. Lot of water is lost by the body of animals by sweating, in the form of urine and evaporation while doing various activities, so to make up for the lost water animals consume a lot of water in the form of drinking.
2. Water keeps the animals fresh and is natural medicine for their many ailments.
3. Water is good solvent and helps in the process of digestion, blood circulation, excretion etc. in the body of organisms.
4. Water is essential for the cleanliness of animals and their surroundings.

Question 6.
What are the three methods of removing germs from natural water. Explain.
Answer:
Three methods to remove germs :

1. By boiling : Boiling kills germs present in water.
2. Adding potassium permaganate: By adding potassium permaganate in the well the germs can be killed.
3. Chlorination : After filtration water is passed through chlorination tank here chlorine kills the germs.

Question 7.
Name the steps involved in the purification of drinking water supplied in cities and towns.
Answer:
The water source for our towns and cities are river, lakes or underground water which contains suspended and dissolved impurities.
To remove these impurities steps involved are :

1. Loading and sedimentation : to settle the suspended impurities to form a sediment for this purpose potash alum is added.
2. Filtration : The water still contains lighter suspended impurities which are removed by filtration through sand and gravel. In cities ground water is drawn from tube well or submurcible pumps which have filters fitted in them. This clear water still contains germs.
3. Chlorination : To kill germs water is passed into chlorination tank where it is treated with chlorine to kill germs.
   Water is now potable i.e. safer for drinking and supplied to homes.

Question 8.
What is the taste of distilled water ? Why is it not potable?
Answer:
Taste of water is tasteless i.e. flat. It is because distilled water does not contain any salt or mineral required for our body. So it is not potable water.

Question 9.
Give reasons :

(a) Ice floats on water.
(b) Marine life is able to survive in colder regions.
(c) Water droplets can be seen outside a chilled water bottle.

Answer:

(a) Water has maximum density at 4°C. Ice is lighter than water therefore it floats on water.
(b) This anomalous property of water enables aquatic plants and animals to survive in colder regions of world because even when the water of ponds, lakes, river freezes it freezes on the top but remains a liquid below ice layer.
(c) Sometimes we see water droplets on the outer surface of the glass containing ice-cold water, this is because the water vapour presents in’air, on coming in contact with the cold glass of water, loses energy and gets converted into liquid state, which we see as water droplets.

EXERCISE-III

Question 1.
Why is water called a universal solvent ?
Answer:
As water can dissolve in most of the substances, solids, liquid and gaseous. Therefore it is called universal solvent.

Question 2.
Define:

(a) Solute
(b) Solvent
(c) Solution
(d) Saturated solutions
(e) Unsaturated solutions

Answer:

(a) Solute : A solute is a substance that dissolves in a medium which can be water or any other substance. Solute is in smaller quantity in a solution.
(b) Solvent : A solvent is a medium in which a solute dissolves. It is in large quantity in a solution. Water is the most common solvent. The other solvents are alcohol, carbon tetrachloride etc.
(c) Solution : A solution is a homogeneous uniform mixture formed by a solute and a solvent.
(d) Saturated solution : When a solution cannot dissolve any more of solute at a given temperature, it is called saturated solution.
(e) Unsaturated solution : A solution that can take up more of the solute at a given temperature, is said to be an unsaturated solution.

Question 3.
State two factors by which solubility of a solute in a solvent can be increased.
Answer:
Two factors are :

1. Stirring.
2. Increase in temperature.

Question 4.
Why do aquatic animals die in boiled water ?
Answer:
Boiled water is deprived of oxygen i.e. there is no oxygen. For want of oxygen animals die in boiled water.

Question 5.
State three differences between water and air.
Answer:
Differences between water and air :
Water :

1. Water is liquid and can exist in three states i.e. solid-ice, liquid-water, gas – vapours.
2. Is a compound.
3. H₂O is its formula.

Air :

1. Air is gaseous and can exist in liquid state when cooled under pressure.
2. Is a mixture.
3. Has no formula.

EXERCISE -IV

Question 1.
State four ways by which water can be conserved.
Answer:
Four ways to conserve water:

1. Do not allow water to drip from defective taps.
2. More dams should be built.
3. More plantation should be done as plants help in bringing rain.
4. Wastage of water should be avoided.
5. Close the tap when you are brushing your teeth. Rather use a mug. Close the tap when you are washing clothes, open the tap only when you need it. Reduce, Reuse and recycle should be our mantra

Question 2.
Explain harvesting of water.
Answer:
When it rains heavily water runs into streets, drains and wasted we can not make use of it. Our purpose is to catch this rain water, store it for future use when we actually need it. For this harvesting of rain water should be done. Rain water is collected from the roofs and verandas of the buildings with the help of pipes and carried to tanks for storage and is used at the time of need when it is not raining.

Question 3.
What are the three main causes of water pollution ?
Answer:
Three main causes of water pollution :

1. Addition of waste products from homes.
2. Addition of waste products from agriculture.
3. Addition of waste from industries.
4. Addition of sewage in water bodies.

Question 4.
State the main steps to be taken to prevent water pollution.
Answer:
Steps to be taken to prevent water pollution :

1. Trees and plants be planted along the banks of rivers and canals.
2. Bathing and cleaning of animals near or in water sources be not allowed.
3. Use of pesticides, insecticides, fungicides and fertilisers should be reduced.
4. The polluted water from industries should be treated first and then discharged into water bodies.
5. Use biodegradable detergents.
6. Water containing sewage should be passed through sewage treatment plants first and then this water should be used for irrigation.
7. Wells should be covered properly and washing and cleaning of clothes, utensils and animals should not be done near the well to keep them clean.

Question 5.
What are the causes of floods and drought?
Answer:
Problems cause by floods :

1. A rise in the level of water in dams, rivers, lakes etc.
2. Heavy rainfall also causes floods.
3. Floods cause extensive damage to crops, property, animal and human life.
4. Crop-fields, villages and”many low-lying areas get submerged under flood water.
5. Rains/floods also affect smaller animals living in the soil.

Problems cause by drought :

1. Crops may die, fodder may become scarce.
2. Living organisms of the soil die.
3. Animals may die, plants and trees will not survive.
4. Soil becomes dry, water level in rivers, lakes, dams etc. may fall. The ground water-level falls.
5. Drought displaces people from a large number of villages and towns.

Question 6.
State some of the ways in which you as an individual can conserve water.
Answer:
The ways in which we can conserve water are as follows :

1. Use a bucket for taking it.
2. Make sure, water does not overflow from overhead tanks of your house.
3. Close the tap when you are brushing your teeth.
4. Wash fruits, vegetable in a bowl of water, rather than under a running tap. Water used for washing vegetables may be used to water plants in the garden.

OBJECTIVE TYPE QUESTIONS

1. Fill in the blanks

(a) Water is a universal; solvent.
(b) Rainwater is the purest form of natural water.
(c) Sand and dust are suspended impurities in water.
(d) Sea water has high concentration of salt (impurity).
(e) Water covers nearly 4 / 5 th of the surface of the earth.
(f) Evaporation of rain water leaves NO residue.
(g) Potash alum is the chemical added to water to remove the tiny suspended particles.
(h) A solution is a uniform mixture of a solute and a solvent.
(i) Ice, water and steam have different physical states but are chemically identical.
(j) Boiling kills most of the germs in water.
(k) The elements present in the molecules of water are hydrogen and oxygen.

2. Write True or False for the following statements

(a) Water is an element.
Answer. False
Correct: Water is aL compound.

(b) Tap water does not contain dissolved impurities.
Answer. False
Correct: Tap water contain dissolved impurities.

(c) Alum is commonly used for removing suspended impurities.
Answer. False
Correct : Alum is commonly used for settling down of suspended impurities.

(d) Distillation is a good method for purifying water for town supply.
Answer. False
Correct : Distillation is a good method-for purifying water for medicinal purpose.

MULTIPLE CHOICE QUESTIONS

Tick (√) the correct alternative from the choice given for the following statements:

1. Water content in human body is

1. 70%
2. 75%
3. 80%
4. 90%

2. The purest form of natural water is

1. seawater
2. river water
3. rainwater
4. lake water

3. When the water vapour changes into tiny droplets of water near the ground, it is called

1. mist
2. dew
3. fog
4. frost

4. Water is a

1. compound
2. element
3. mixture
4. none of the above

5. Common salt is obtained from sea water by

1. distillation
2. crystallisation
3. evaporation
4. sublimation

6. Jaundice affects

1. heart
2. lungs
3. liver
4. kidney

7. Chlorination of water is done

1. to kill the germs
2. to remove the suspended impurities
3. to remove the dissolved impurities
4. none of the above

Selina Concise Chemistry Class 6 ICSE Solutions – Pure Substances and Mixtures ; Separation of Mixtures

ICSE SolutionsSelina ICSE SolutionsML Aggarwal Solutions

APlusTopper.com provides step by step solutions for Selina Concise ICSE Solutions for Class 6 Chemistry. You can download the Selina Concise Chemistry ICSE Solutions for Class 6 with Free PDF download option. Selina Publishers Concise Chemistry for Class 6 ICSE Solutions all questions are solved and explained by expert teachers as per ICSE board guidelines.

Selina Class 6 Chemistry ICSE SolutionsPhysicsBiologyMathsGeographyHistory & Civics

Selina Concise ICSE Solutions for Class 6 Chemistry Chapter 5 Pure Substances and Mixtures ; Separation of Mixtures

POINTS TO REMEMBER

1. Element is made up of atoms of the same kind i.e. atoms of gold are similar in every respect i.e. have same melting point and same boiling pts. same colour, mass and odour.
2. Molecules of a compound are similar in every respect i.e. same composition taste, colour and odour i.e. compound water is made up of similar molecules.
3. Pure substances are either elements or compounds have definite set of properties.
4. Pure substances are required to maintain good health of human beings. Chemists, technologists and scientists need pure substances for manufacture of medicines, chemicals in industry and for scientific purposes.
5. Milk, air, bronze, sugar and water solution, salt and water solution, salt, iron filings and sand, petrol are all mixtures.
6. Mixtures, two or more components mixed in any ratio, undergo ho chemical change and retain their individual properties, can be separated by simple physical methods.
7. Mixture has no formula, no change in volume, no change in mass, energy is neither needed nor produced.
8. Mixture is impure substance.
9. Homogeneous mixture : in which constituents are uniformaly distributed throughout its volume, e.g. salt and water solution.
10. Heterogeneous mixture: The components are not uniformly distributed through its volume and components can be easily seen separately, e.g. water, oil solution.
11. Milk is emulsion but this mixture cannot be separated. Oh shaking a mixture of mustard oil and water vigorously mixture becomes milky and is called an emulsion after some time water and oil get separated.
12. Alloys are homogeneous mixtures of metal with metals or metal with non-metals.
13. Salt and sand can be seperated by solvent extraction method where one of the solid component (salt) is soluble in liquid.
14. Coagulation or loading : When alum (a chemical) is added to mixture dissolves in water and form clusters with clay and fine dust particles making them heavier, increases the rate of sedimentation.
15. Centrifugation method is used to separate solids from liquids where mixture is homogeneous. Cream is separated being lighter on churning floats on a liquid (milk).
16. Loading : The process of adding a chemical substance to help the suspended solid particles in liquid to form a sediment is called loading.

EXERCISE – I

Question 1.
Select homogeneous and heterogeneous mixtures from the following:
Salt solution, petrol and water, sand and charcoal, alcohol and water, air dissolved in water, air, sea water, fruit juices, mist, brass.
Answer:
Homogeneous mixture : Salt solution, alcohol and water, air dissolved in water, sea water, brass.
Heterogeneous mixture: Sand and charcoal, air, fruit juice, mist, petrol and water.

Question 2.
Define the following :

(a) Pure substance
(b) Impure substance
(c) Alloy
(d) Solution
(e) Heterogeneous mixture
(f) Homogeneous mixture

Answer:

(a) Pure substance : “Pure Substance is either element or compound. It contains the same kind of atom or molecules and has a definite set of physical and chemical properties.”
(b) Impure substance : “A substance in which some other substances are also present in smaller or larger amounts is called an impure substance. Mixtures are impure substance.
Example of impure substance is air.
(c) Alloy : “A homogeneous solid mixture of two or more metals or a metal and a non-metal is called an alloy.”
(d) Solution : “The homogeneous mixture of water (or any other solvent) and a substance soluble in it is called a solution.”
(e) Heterogeneous mixture : “A mixture in which the components are not uniformally distributed through its volume and can be easily seen separately is called heterogeneous mixture.”
(f) Homogeneous mixture : “A mixture in which its constituents are uniformly distributed throughout its volume and cannot be seen separately is called a homogenous mixture.”

Question 3.
List four characteristics of a mixture.
Answer:
Four characteristics of a mixture :

1. Mixture has no fixed composition.
2. To form a mixture energy is neither produced nor evolved.
3. Mixture has no fixed melting’point and boiling points.
4. Mixture retain the properties of its components.
5. Components of mixtures can be seperated by simple physical methods.

Question 4.
Give reasons :

(a) Why do sugar and water retain their individual properties in a sugar solution ?
(b) Why do petrol and water form a heterogeneous mixture ?
(c) Why sulphur does dissolve when carbon disulphide is added to a mixture of iron and sulphur but not when it is added to iron sulphide ?

Answer:

(a) As sugar solution is a mixture and mixtures has not any specific set of properties. They show the properties of the individual components from which they are formed.
(b) Petrol and water forms a heterogeneous mixture as its constituents can be seen separately and are not uniformly distributed throughout its volume.
(c) Sulphur has the property to dissolve in carbon disulphide whereas iron does not dissolve and retains its individual property. However, On heating Iron and Sulphur, they chemically combined forming Iron Sulphide. In this, Iron and Sulphur particles do not exists separately as such they loose their individual property.

Question 5.
Give two examples for each of the following types of mixture.

(a) solid-solid
(b) solid-liquid
(c) liquid-gas
(d) gas-gas

Answer:
Two examples of :
(a) solid – solid –

1. sand and sugar
2. sand and iron filling.

(b) solid – liquid –

1. salt and water
2. charcoal and water.

(c) liquid – gas –

1. coca cola
2. mist.

(d) gas – gas –

1. air
2. helium and hydrogen in air balloon,
3. perfumes and air.

Question 6.
Name the components present in the following mixtures:

(a) Brass
(b) Duralumin
(c) Tap water
(d) Bronze
(e) Crude petroleum oil .

Answer:

(a) Brass → Copper and Zinc.
(b) Duralumin → Aluminium + Copper with little manganese and magnesium.
(c) Tap water → air, dissolved salts.
(d) Bronze → Copper, Tin and zinc.
(e) Crude petroleum oil → petrol, kerosene, diesel, LPG, mixed with salt, water and earth particles.

Question 7.
State:

(a) Three differences between water and air.
(b) Four differences between compounds and mixtures.

Answer:
(a)
Water :

1. The components of water are hydrogen and oxygen which are chemically combined in a fixed ratio of 1 : 8 by mass.
2. The chemical composition of water remains same from whatever source it is obtained.
3. The properties of water are completely different from the properties of elements from which it is formed i.e. hydrogen and oxygen.
4. Energy change occurs in the formation of water.
5. A molecule of water is represented by a definite formula H₂O.

Air :

1. The main components of air are nitrogen, oxygen, carbon -dioxide, water vapour which are not chemically combined.
2. The composition of air varies from place to place. During rainy season the air becomes humid due to presence of more water vapour. Some impurities like sulphur dioxide, hydrogen sulphide etc. also changes its composition at some places.
3. The components of air retain their individual properties but not air.
4. No energy change occurs when components of air are mixed together.
5. Air cannot be represented by any chemical formula.

(b)
Compound :

1. A compound is a pure substance.
2. Compounds are always homogeneous.
3. A compound has a fixed composition, i.e., it is formed when two or more pure substances chemically combine in a definite ratio by mass.
4. Formation of a compound involves change in energy.
5. Compounds have specific set of properties.
6. Components of compounds can be separated only by complex chemical processes.

Mixture :

1. A mixture is an impure substance.
2. Mixtures may be homogeneous or heterogeneous.
3. A mixture has no fixed composition, i.e., it is formed by mixing two or more substances in any ratio without any chemical reaction.
4. Formation of a mixture does not involve any change in energy.
5. Mixtures do not have any specific set of properties.
6. Components of mixtures can be separated by simple physical methods.

EXERCISE – II

Question 1.
Define:

(a) Filtration
(b) Sublimation
(c) Evaporation
(d) Crystallisation
(e) Miscible liquids
(f) Immiscible liquids

Answer:

(a) Filtration : The process of separating solid particles from liquid by allowing it to pass through a filter paper is called filtration.
(b) Sublimation : The process in which a solid changes directly into its vapours on heating is called sublimation.
(c) Evaporation : Is the process of converting a liquid into its vapours state either by exposing it to air or by heating.
(d) Crystallisation : Evaporation of liquid from a homogeneous liquit-solid mixture and collecting solid in the form of crystals is called
crystallisation.
(e) Miscible liquids: Homogeneous liquid-liquid mixtures are called miscible liquids.
(f) Immiscible liquids : Heterogeneous liquid-liquid mixtures are called immiscible liquids.

Question 2.
Why do we need pure substances?
Answer:
We need pure substances because of the following reasons:

1. A pure substance has a fixed melting and fixed boiling point.
2. A pure substance has its characteristic taste, colour and odour.
3. Pure substances can not be broken further into more simple substances by any physical means.

Question 3.
Give one example for each of the following types of mixtures.

(a) Solid-solid heterogeneous mixture
(b) Solid-liquid heterogenous mixture
(c) Solid-liquid homogeneous mixture

Answer:

(a) Iron and sulphur.
(b) Sand and water, rice and water.
(c) Sugar from its solution in water.

Question 4.
Name the process by which the components of following mixtures can be separated.

(a) Powdered glass and sugar
(b) Chalk powder and iron filings
(c) Chaff and grain
(d) Salt and water
(e) Wheat and sugar
(f) Sand and camphor
(g) Sugar and water

Answer:

(a) FILTRATION : Glass and sugar on dissolving in water and filtering, glass separates out as residue on the filter paper. Filtrate of sugar solution is heated to remove water by evaporation, sugar is collected as crystals.
(b) MAGNETIC SEPERATION : With the help of a magnet, iron filings can be separated leaving behind chalk powder.
(c) WINNOWING : It separates chaff (lighter) from heavier grains in two different heaps.
(d) EVAPORATION : This method is used to separate the components of a homogeneous solid-liquid mixture, like salt from sea water. Sea water is collected in shallow beds and allowed to evaporate in the sun. When all the water is evaporated, salt is left behind.
(e) EVAPORATION : Wheat and sugar are put in water in a beaker. Sugar dissolves and mixture is passed through strainer and separated and dried. Sugar is obtained by evaporating sugar solution.
(f) SUBLIMATION : Camphor sublimes on heating leaving behind sand.
(g) CRYSTALLISATION : Pure sugar is obtained from its solution in water by the process of crystallisation. At first the sugar solution is heated to evaporate Water at a faster speed. When very less of water is left the solution is cooled. On cooling sugar dissolved in it starts separating out in the form of crystals.

Question 5.
Name:

(a) two substances which can sublime
(b) two substances soluble in water
(c) two substances insoluble in water
(d) four substances that can be used as filters.

Answer:

(a) Camphor and Naphthalene
(b) Sugar and salt (NaCl)
(c) Sand and chalk powder.
(d) (i) Filter paper, (ii) A bead of sand, (iii) Charcoal, (iv) A piece of muslin cloth.

Question 6.
Give reasons :

(a) Sand and saw dust cannot be separated by hand picking.
(b) Magnet is used to separate a mixture of iron and sulphur.
(c) Alum is used in purification of river water.

Answer:

(a) Because in hand picking method substances should be large enough in size to be recognized and picked out by hand but sand and saw dust particles are very small in size so they can’t be picked by hand. It can be separated by filtration.
(b) Mixtures of iron and sulphur can be separated by moving a magnet over them. Iron gets attached to the magnet is separated.
(c) Water from a river, pond or lake contains very fine clay particles. To make them settle at a faster rate, a chemical substance called alum in powdered form is added to such mixtures. It dissolves in water and forms clusters with clay and dust particles making them heavier and increasing the rate of sedimentation.

OBJECTIVE TYPE QUESTIONS

1. Fill in the blanks

(a) The substances that make a mixture are called its constituents or components.
(b) Evaporation or crystallisation is a process to separate solids dissolved in liquids.
(c) Mist is a heterogeneous (liquid in gas) mixture of droplets of water and air.
(d) Clay is separated from water by the method of loading and decantation.
(e) When cereals are washed before cooking, water is separated from the cereals by decantation.
(f) Crystallisation is a process to obtain a very pure form of a solid dissolved in a liquid.
(g) Ammonium chloride can be separated from common salt by the method of sublimation.
(h) The solid particles which remain on the filter paper are called residue and the liquid which passes through it is called filtrate.
(i) The process of transferring the clear liquid layer above the solid particles which settle at the bottom of the container is known as decantation.
(j) Filtration is a method used for the separation of an insoluble solid from a solid-liquid mixture.

2. Write “true” or “false” for the following statements 

(a) A pure substance consists of only one kind of atom or molecule.
Answer. True

(b) Common salt is separated from its solution in water by decantation.
Answer. False
Correct : Common salt is separated from its solution in water by evaporation.

(c) Winnowing is a process to remove small stones from grains.
Answer. False
Correct : Winnowing is a process to remove husk from grains.

(d) Jewellery gold is a homogeneous mixture of metals.
Answer. False
Correct : Jewellery gold is a heterogeneous mixture of metals.

(e) Air can be separated from water by filtration.
Answer. False
Correct : Air can be separated from water by heating.

(f) Salt and air dissolved in water add taste to water.
Answer. True

(g) Steel is an alloy of iron and aluminium.
Answer. False
Correct: Steel is an alloy of iron and carbon.

MULTIPLE CHOICE QUESTIONS

Tick (√) the correct alternative from the choice given for the following statements:

1. The process of adding a chemical substance to help the suspended solid particles to deposit as sediment fastly is called

1. loading
2. sedimentation
3. decantation
4. filtration

2. Salt is separated from sea water by

1. evaporation
2. sublimation
3. crystallisation
4. filtration

3. A mixture of mustard oil and water forms

1. a compound
2. a homogeneous mixture
3. an alloy
4. a heterogeneous mixture

4. A heterogeneous mixture is

1. made up of only one kind of atom
2. made up of only one kind of molecule
3. made up of different kinds of atoms and molecules.
4. that looks uniform

5. Example of a homogeneous mixture is

1. distilled water
2. tap water
3. sand and water
4. sawdust and water

6. A set of mixture is

1. gold, common salt, water, alloy
2. alloy, ink, honey, icecream
3. alloy, mercury, air, sea water
4. milk, duralumin, brass, silver

7. A gas dissolved in a liquid can be separated by :

1. filtration
2. boiling
3. using magnet
4. by crystallisation

8. Copper is not a part of the alloy :

1. brass
2. bronze
3. steel
4. duralumin

9. Which is not a mixture?

1. sugar solution
2. tap water
3. milk
4. distilled water

10. Give one word name for the following

(a) The solid which is left on the filter paper after filtration residue.
(b) The solid particles which separate out from the solution on slow evaporation crystals.
(c) The solid particles that settles at the bottom of the beaker in a heterogeneous mixture of a solid and a liquid decantation.
(d) The clean liquid which is poured out after sedimentation supernatant liquid.
(e) The technique used to separate the light particles from heavy particles using the flow of wind winnowing.