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Chapter 8


Class 8 - Concise Chemistry Selina

Exercise I

Question 1

Name the four main sources of water.


The four main sources of water are:

  1. Ocean and sea water
  2. River and lake water
  3. Spring and well water
  4. Rainwater

Question 2

State the importance of water cycle in nature.


The importance of water cycle in nature are:

  1. It helps in regulating the climate on the earth.
  2. It makes water available in various forms on the earth.

Question 3

Why is water very precious for all living beings?


Water is very precious for all living beings due to the following reasons:

  1. Water is the source of life. It is the most essential substance for the existence of life.
  2. It is required for everyday uses like drinking, cleaning, bathing, washing etc.
  3. It provides medium for all biochemical reactions inside our bodies to take place.
  4. It regulates our body temperature through sweating and evaporation.
  5. It is required for growing crops
  6. It provides nutrients to aquatic plants and animals.
  7. It helps in germination of seeds, growth of plants, photosynthesis.
  8. It is used in various manufacturing processes.
  9. It is used to generate electricity.
  10. It plays an important role in controlling our earth's climate.

Question 4

Name the two gases from which water is formed. What is the chemical composition of these two gases in water? Give the molecular formula of water.


Water is formed from hydrogen and oxygen gases. The chemical composition of hydrogen and oxygen gases is 1:8 ratio by mass or 2:1 by volume in water. The molecular formula of water is H2O.

Question 5

How does anomalous expansion of water help the aquatic organisms in cold climates?


In lakes, as the temperature drops, the surface water cools, sinks and mixes with the deeper water, this process continues until the temperature reaches 4°C. Beyond that point, the surface water expands, becomes lighter and it does not sink. So, further cooling takes place only at the surface while temperature of lower layers of water does not change. The water at the surface freezes to form a layer of ice that insulates the deeper water and allows aquatic organisms to survive in the water beneath the ice.

Question 6a

Give reasons:

Water is used as a cooling agent.


Water has a high specific heat i.e., it needs a large amount of heat to become hot. Thus, water can absorb a lot of heat without much rise in its temperature. Hence, it is used as an excellent cooling agent.

Question 6b

Give reasons:

Water pipes burst in severe winters.


During severe winters when temperature starts falling below 4°C, water in pipelines expands (due to property of anomalous expansion of water) and it exerts large pressure on the pipes causing them to burst.

Question 6c

Give reasons:

It is difficult to cook in hills as compared to plains.


The boiling point of water decreases with a decrease in pressure. Therefore, water boils at a lower temperature (i.e., less than 100°C) in hilly areas where the atmospheric pressure is lower as compared to the plains. Due to this it takes a longer time to cook food making cooking difficult in hills.

Question 6d

Give reasons:

Ice floats on water.


Density of ice is less than the density of water, so ice floats on water.

Question 6e

Give reasons:

Sea water does not freeze at 0°C.


The presence of impurities in water lowers its freezing point. Sea water contains salt which lowers the freezing point of water. So sea water does not freeze at 0°C.

Question 7

What is the effect on boiling point of water when:

(a) pressure is increased?

(b) impurity is added?


(a) When pressure is increased, boiling point of water is increased.

(b) When impurity is added, the boiling point of water increases.

Exercise II

Question 1

Explain the terms:

(a) Solution

(b) Solute

(c) Solvent


(a) A solution is a homogeneous mixture of two or more substances whose composition can be varied.

(b) A solute is a substance that dissolves in other substances. It is in a smaller quantity in the solution.

(c) A solvent is a medium in which a solute dissolves. It is in a larger quantity in a solution.

Question 2

What is meant by:

(a) Unsaturated

(b) Saturated and

(c) Supersaturated solutions.


(a) A solution in which more of the solute can be dissolved at a given temperature is called an unsaturated solution.

(b) A solution that cannot dissolve any more of the solute at a given temperature is called a saturated solution.

(c) A saturated solution that contains more solute than it can hold at room temperature is called supersaturated solution.

Question 3

How do the solubility of a solid and a gas gets affected by:

(a) Increase in temperature?

(b) Increase in pressure?


(a) Solubility of a solid generally increases with an increase in temperature.
Solubility of a gas decreases with an increase in temperature.

(b) Pressure has practically no effect on the solubility of a solid in water.
In the case of gases, the amount of a gas dissolved in water increases with an increase in pressure and decreases with a decrease in pressure.

Question 4

Differentiate between:

(a) Solution and suspension

(b) Suspension and colloid


(a) Difference between Solution and suspension:

Solution is a homogeneous mixture.Suspension is a heterogeneous mixture.
Particle size less than 10-10 m.Particle size greater than 10-7 m.
It is transparent.It is opaque.
Particles of solute are invisible.Solute particles are visible.
Particles of solute do not settle.Solute particles settle at the bottom of the container.
Solute particles cannot be filtered. The solution passes easily through a filter paper.Solute particles do not pass through a filter paper.

(b) Difference between Suspension and colloid:

Particle size greater than 10-7 m.Particle size between 10-10 - 10-7 m.
It is opaque.It is translucent.
Solute particles are visible.Solute particles can be seen with the help of a powerful microscope.
Solute particles settle at the bottom of the container.Solute particles do not settle.
Solute particles do not pass through a filter paper.Solute particles pass easily through ordinary filter papers but do not pass through ultra fine filters or nano filters.

Question 5

Define water of crystallization. Give two examples with formulae.


The fixed amount of water which is in a loose chemical combination with one molecule of a substance in its crystal form is called water of crystallization.

The two examples are:

Common nameChemical nameFormula
Blue vitriolCopper sulphate pentahydrateCuSO4.5H2O
Washing sodaSodium carbonate decahydrateNa2CO3.10H2O

Question 6

Give two examples for each of the following:

  1. Hydrated substances
  2. Crystalline anhydrous substances
  3. Drying agents
  4. Deliquescent substances
  5. Efflorescent substances
  6. Colloids
  7. Solvents other than water


  1. Blue vitriol (CuSO4.5H2O), Washing soda (Na2CO3.10H2O)
  2. Common salt (NaCl), Potassium nitrate (KNO3)
  3. Quicklime (CaO), Anhydrous calcium chloride (CaCl2)
  4. Caustic soda (NaOH), Magnesium chloride (MgCl2)
  5. Washing soda (Na2CO3.10H2O), Glauber's salt (Na2SO4.10H2O)
  6. Milk, blood
  7. Alcohol, Carbon tetrachloride

Question 7

What do you observe when:

(a) Blue vitriol is heated?

(b) Washing soda is exposed to air?

(c) Blue litmus solution is added to water?


(a) When blue vitriol is heated, it loses its water of crystallisation. The blue crystalline solid of copper sulphate turns into white amorphous (powder) solid and drops of colourless liquid (which is water) condense on the upper cooler parts of the test tube.

(b) When washing soda is exposed to air the white crystalline substance changes into white powder.

(c) There is no change in colour of blue litmus solution when added to water as pure water is neutral.

Question 8a

Give reason:

Silica gel pouches are kept in unused water bottles.


Silica gel is a drying agent which absorb moisture and keep things dry. So silica gel pouches are kept in unused water bottles.

Question 8b

Give reason:

Table salt becomes moist during rainy season.


Table salt contains impurities like magnesium chloride and calcium chloride which are deliquescent. They absorb water and become moist during rainy season.

Question 8c

Give reason:

On opening a bottle of cold drink, a fizz sound is heard.


Cold drinks bottles contain dissolved carbon dioxide under high pressure. On opening the bottle, the pressure decreases and the gas releases out producing a fizz sound.

Question 9

Give balanced chemical equations for the reaction of water with:

(a) Sodium

(b) Iron

(c) Carbon dioxide

(d) Sodium oxide


(a) 2Na + 2H2O ⟶ 2NaOH + H2 (g)

(b) 3Fe + 4H2O ⇌ Fe3O4 + 4H2 (g)

(c) CO2 + H2O ⟶ H2CO3

(d) Na2O + H2O ⟶ 2NaOH

Question 10

What is metal activity series?


The reaction of metals with water under different conditions indicates the reactive nature of metals and enables us to arrange the metals on the basis of their reactivity which is called metal activity series.

Question 11

Name the gas produced when:

(a) steam is passed over hot coke.

(b) chlorine is dissolved in water and exposed to sunlight.

(c) a piece of calcium is added to water.

(d) when fossil fuel is burnt.


(a) Water gas [CO + H2]

(b) Oxygen gas (O2)

(c) Hydrogen gas (H2)

(d) Carbon dioxide gas (CO2)

Exercise III

Question 1


(a) Soft water

(b) Hard water


(a) Water which forms a lather with soap easily is called soft water.

(b) Water which does not forms a lather with soap easily rather forms a white sticky scum or precipitate is called hard water.

Question 2(a)

Name the compounds responsible for:

(i) temporary hardness

(ii) permanent hardness of water.


(i) Bicarbonates of calcium and magnesium

(ii) Sulphates and chlorides of calcium and magnesium

Question 2(b)

Suggest one method for the removal along with the reactions for

(i) temporary hardness

(ii) permanent hardness of water


(i) Boiling helps to remove the temporary hardness of water. When temporary hard water is boiled, the bicarbonates of calcium and magnesium break up to form their respective insoluble carbonates. These can be filtered out and the water becomes soft.

Ca(HCO3)2 boiled\xrightarrow{\text{boiled}} CaCO3 + H2O + CO2 (g)

Mg(HCO3)2 boiled\xrightarrow{\text{boiled}} MgCO3 + H2O + CO2 (g)

(ii) The permanent hardness of water is removed when it is treated with a small quantity of sodium carbonate. It reacts with soluble chlorides and sulphates of calcium and magnesium to form respective insoluble carbonates. These are filtered out and water becomes soft.

Na2CO3 + CaSO4 ⟶ CaCO3 ↓ + Na2SO4

Na2CO3 + MgCl2 ⟶ MgCO3 ↓ + 2NaCl

Question 3

What are the main causes of water pollution? How can it be controlled?


The main causes of water pollution are:

  1. Chemical wastes from industrial and agricultural processes.
  2. Thermal wastes from nuclear and thermal power plants.
  3. Sewage and garbage disposed into water bodies.

It can be controlled by the following ways:

  1. We should spread awareness and promote access to sanitation in rural areas and city slums to prevent open defecation.
  2. Domestic sewage should be treated before being discharged into water bodies.
  3. Waste products of industries should be treated before being discharged into water bodies.
  4. Use non-biodegradable materials such as detergents, pesticides and polythene responsibly to avoid pollution of water and the environment.
  5. Purification of water bodies should be done from time to time.
  6. Washing of clothes, utensils, bathing of animals and human beings should be avoided in or near water sources.
  7. Dead bodies of animals should be disposed off in a hygienic way.

Question 4

Name three water borne diseases.


Typhoid, cholera, hepatitis are the three water borne diseases.

Question 5(a)

Give reasons:

Alcohol is mixed with water and is used in car radiators.


Alcohol is mixed with water and is used in car radiators to prevent it from freezing in cold weather because presence of impurities in water lowers the freezing point.

Question 5(b)

Give reasons:

Icebergs float on ocean water.


Density of ice is less than density of water, so icebergs float on water.

Question 5(c)

Give reasons:

Carbonated drinks are bottled under high pressure.


The amount of gas dissolved in water increases with an increase in pressure. That is why carbonated drinks containing carbon dioxide are bottled under high pressure as they contain a large amount of gas dissolved in them.

Objective type questions

Question 1

Fill in the blanks:

(a) Water has ..................... density and ................. volume at 4°C.

(b) Freezing mixture contains .................. and ................ .

(c) The solubility of a gas in water ................. with rise in temperature and ....................... with rise in pressure.

(d) ................... is the purest form of natural water.

(e) Use of excessive .................. by farmers causes water pollution.

(f) Boiling removes the ................... hardness of water.

(g) Water turns the colour of anhydrous copper sulphate ................... .

(h) The sticky substance formed when soap is added to hard water is ...................... .


(a) maximum, minimum

(b) salt, ice

(c) decreases, increases

(d) Rain water

(e) fertilizers

(f) temporary

(g) blue

(h) scum

Question 2

Match the statements in column A with that in column B.

Column AColumn B
deliquescent substancesodium bicarbonate
hygroscopic substancemagnesium chloride
efflorescent substanceconc. sulphuric acid
substance causing temporary hardnesswashing soda


Column AColumn B
deliquescent substancemagnesium chloride
hygroscopic substanceconc. sulphuric acid
efflorescent substancewashing soda
substance causing temporary hardnesssodium bicarbonate

Question 3

Give one word/words for the following statements:

(a) Water fit for human consumption.

(b) The harmful substances dissolved in water.

(c) The change of states of water from one form to another.

(d) The gaseous form of water found in air.

(e) A mixture of common salt and ice.

(f) A substance which does not contain water in its chemical structure.

(g) A property due to which a substance absorbs water without dissolving.

(h) Water molecules in loose chemical combination with other substances.


(a) Potable water

(b) Impurities

(c) Water cycle

(d) Water vapours

(e) Freezing mixture

(f) Anhydrous substance

(g) Hygroscopy

(h) Water of crystallisation

Question 4(a)

Two gases found dissolved in natural water are

  1. oxygen and carbon dioxide
  2. hydrogen and oxygen
  3. sulphur dioxide and hydrogen
  4. chlorine and ammonia


oxygen and carbon dioxide

Reason — Rain water is the purest form of natural water which contains dissolved gases like oxygen, carbon dioxide, sulphur, nitrogen, oxides of nitrogen.

Question 4(b)

Temporary hardness of water can be removed by

  1. filtering
  2. boiling
  3. loading
  4. none of the above



Reason — Boiling helps to remove the temporary hardness of water. When temporary hard water is boiled, the bicarbonates of calcium and magnesium break up to form their respective insoluble carbonates. These can be filtered out and the water becomes soft.

Question 4(c)

The ultimate source of all water on the earth is

  1. oceans and seas
  2. springs and wells
  3. rivers and lakes
  4. rain



Reason — Rainwater is the ultimate source of all water on the earth.

Question 4(d)

Colloids have the particle size ranging between

  1. 10-7 to 10-10 m
  2. 10-10 to 10-12 m
  3. 10-7 to 10-5 m
  4. 10-12 to 10-15 m


10-7 to 10-10 m

Reason — Colloids have the particle size ranging between 10-7 to 10-10 m.