WATER

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Afzala janpati biraul Darbhanga, City br

NEW TRENDS IN ICSE CHEMISTRY - 8

CHAPTER - 8. Water

Tick (✔) The correct choice amongest the following

1. Water which is free from impurity and fit for human consumption is known as :(i) Hard water(ii) Potable water(iii) Soft water(iv) Clean water

2. Organic waste in water comes mainly from:(i) Industries(ii) Chimneys(iii) Sewage(iv) Vehicles

3. Alum is used in the process of :(i) Sedimentation(ii) Loading(iii) Filteration(iv) Aeration

4. Chlorination is commonly done by using :(i) Chlorine gas(ii) Hydrogen chloride(iii) Sulphuryl chloride(iv) Bleaching powder

5. Heat treatment given to water kills germs. The process is :(i) Aeration(ii) Filteration(iii) Chlorination(iv) Boiling

6. Which plants use dissolved gases in water?(i) Aerial plants(ii) Aquatic plants(iii) Submerged plants(iv) Desicated plants

7. An inflammable gas which is present in water:(a) Hydrogen(b) Oxygen(c) Water vapours(d) Steam

8. An apparatus used for electrolysis of water: (a) Electrolytic cell(b) Hoffmans cell(c) Voltametric cell(d) Hoffmans voltameter

9. Gas which is collected at cathode when water is electrolysed :(a) Hydrogen(b) Oxygen(c) Chlorine(d) None of these

10. Water is formed when plants and animals :(a) Respire(b) Digest(c) Move(d) Sleep

11. Pure water boils :(a) Below 100°C(b) Above 100°C(c) At 100°C(d) None of these

12. Chloride and sulphates of calcium and magnesium cause :(a) temporary hardness(b) permanent hardness(c) softness of water(d) none of these

13. Temporary hardness can be removed by :(a) treating with acids(b) treating with bases(c) addition of salts(d) boiling

14. Water that is fit for drinking is called :(a) potable water(b) non-potable water(c) mineral water(d) acidic water

15. Cholera and diarrohea are :(a) air borne diseases(b) water borne diseases(c) microbial diseases(d) none of these

16. Plan launched for purification of river water:(a) Beas Action Plan(b) Ganga Action Plan(c) Satluj Action Plan(d) River Action Plan

17. Sacrificial protection is the coating of any substance with :(a) Nickel(b) Zinc(c) Manganese(d) Copper

18. Chemical added during the process of loading is :(a) Aluminium nitrate(b) Aluminium oxide(c) Alum(d) None of these

19. The density of water is maximum at:(a) 0°C(b) 4°C(c) -A°C(d) -1°C

20. Permanent hard water forms :(a) Good lather with soap solution(b) Sticky scum with soap solution(c) Air bubbles in soap solution(d) Ions of calcium and magnesium

21. Latent heat of vaporisation is :(a) 2257 J/g(b) 2268 J/g(c) 226 J/g(d) 126 J/g

Fill in The Blanks

1. .................... plants use dissolved gases in water.

2. .................... reactions take place in our body.

3. Water is used as a .................... in car radiators.

4. Water helps in transportation of .................... .

5. .................... absorb the minerals dissolved in water.

6. At low pressure, the boiling point is also .................... .

7. Presence of .................... freezing point of a liquid.

8. Water shows anomalous or unusual behaviour when heated or cooled between  .................... and .................... .

9. Density of ice is minimum at ....................

10. If volume is decreased, the density ....................

11. Water is .................... conductor of heat.

12. Water is a .................... solvent as it dissolves a large variety of substance.

13. Water turns .................... cobalt chloride .................... .

14. Metallic oxides react with cold water to form their respective .................... .

15. Sulphur dioxide dissolves in cold water to form .................... .

16. Sulphur trioxide dissolves in cold water to form .................... .

\

17. Metals like .................... and .................... do not react with water.

18. Reactive metals like sodium react with water liberating .................... gas.

19. The use of excessive .................... and pesticides should be avoided.

20. Leakage of petroleum oil during drilling and shipping operations is commonly termed as .................... .

21. Water that lather with soap easily without forming any sticky scum is called .................... .

22. .................... and .................... are water borne diseases.

23. Due to thermal pollution there is a rapid growth .................... of which decreases the amount of oxygen dissolved in water.

24. Ice floats on water. This means that in ice the molecules have .................... space between them.

25. Water has the maximum density at .................... °C.

26. Water can dissolve many substances in it, due to this special property it is called .................... solvent.

27. The volume of water .................... during solidification.

28. In hot water, the amount of dissolved .................... decreases.

Write True or False for each statment.Rewrite the Statement Correctly

1. Distilled water is fit for drinking.

2. Sea water is saline due to high salt content.

3. The process in which light particles become heavy due to chemicals and settle down is known as sedimentation.

4. Body fluids do not contain water.

5. Impurities present in water make it fit for drinking.

6. The heat capacity of water is high and its heat conductivity is poor.

7. Sea water contains considerable amount of salt dissolved in it, so it is highly saline.

8. Liquid water is lighter than solid ice.

9. Cholera and diarrohea are waterborne diseases.

10. Pure water has a good taste.

 Mark(s)

11. Detergents are biodegradable.

Match The Columns

1. 1.Water borne disease2.Heavy industries3.Spring water4.Rain water Thermal pollutionSoft waterHard waterTyphoid

2. 1.Human body2.Potable water3.Watermelon4.Purest form of water 90% waterRain waterDrinking water70% water

Answer The Followings Questions

1. Name three major water pollutants giving their sources.

2. What is the density of water ?

3. What happens to the volume when water freezes ?

4. Name two water borne diseases.

5. How does fishes and aquatic animals survive when the pond gets covered with thick ice ?

6. Natural water is never pure. Why ?

7. Why is hard water not preferred for washing and in industry ?

8. What role water plays in human body ?

9. How loading helps to remove suspended impurities ?

10. How would you conclude that burning of hydrogen produces water ?

11. Explain how temporary and permanent hardness of water is removed.

12. How water is purified for supply in cities ?

Select the Odd and Out Giving reason

1. Sedimentation, Flocculation, Filteration, Chlorination, Nitrification.

2. Potable water, Well water, Rain water, Distilled water, Sea water.

3. Rain water, River water, Lake water, Sea water, Spring water.

4. Sedimentation, Filtration, Chlorination, Boiling or Distillation, Gravitation.

5. Diarrhoea, Dysentery, Gastroenteritis, Jaundice, Anaemia.

Answers

Tick (✔) The correct choice amongest the following

1. (ii) Potable water

2. (iii) Sewage

3. (i) Sedimentation

4. (i) Chlorine gas

5. (iv) Boiling

6. (ii) Aquatic plants

7. (a) Hydrogen

8. (d) Hoffmans voltameter

9. (a) Hydrogen

10. (a) Respire

11. (c) At 100°C

12. (b) permanent hardness

13. (d) boiling

14. (a) potable water

15. (b) water borne diseases

16. (b) Ganga Action Plan

17. (b) Zinc

18. (c) Alum

19. (a) 0°C

20. (b) Sticky scum with soap solution

21. (d) 126 J/g

Fill in The Blanks

1. Aquatic

2. Metabolic

3. coolant

4. nutrients

5. Roots

6. lowered

7. impurities

8. 0°c, 4°C

9. 0° c

10. increases

11. bad

12. universal

13. blue, pink

14. hydroxide

15. sulphurous acid

16. sulphuric acid

17. gold, silver

18. hydrogen

19. fertilizers

20. oil slick

21. soft water

22. Cholera, diarrohea

23. algae

24. intermolecular

25. 4

26. universal

27. increase

28. gases

Write True or False for each statment.Rewrite the Statement Correctly

1. False

2. True

3. False

4. False

5. False

6. True

7. True

8. False (Liquid water is heavier than solid ice)

9. True

10. False (Pure water has a flat taste)

11. False (Detergents are non-biodegradable)

Match The Columns

1. 1.Typhoid

2.Thermal pollution

3.Hard water

4.Soft water

2. 1.70% water

2.Drinking water

3.90% water

4.Rain water

Answer The Followings Questions

1. • Synthetic detergent -----> Household waste

• Pesticides -----> Agricultural fields

• Lead and Mercury -----> Industrial waste

2. lg/cc at 4°C

3. Volume increases when water freezes.

4. Jaundice, cholera

5. Upper layer of ice acts as an insulator and protects them from further cold.

6. Because water is universal solvent, it dissolves something of everything.

7. Because it doesn't form lather with soap and it deposits in the boiler tubes as hard as stone-like layer.

8. All metabolic reactions take place in water. It helps in transportation of nutrients.

9. It is the process in which chemicals like alum is added to coagulate the floating matter so that impurities become heavier and settle down.

10. 2H2 + 02 ----> H2O

Because water turns anhydrous copper sulphate blue.

11.

12. Two main steps as involved in purification of water:

I. Removal of suspended impurities.

II. Destruction of harmful germs and bacteria.

I. Removal of suspended impurities :

1. Sedimentation : The process in which heavy particles like clay etc. settle down at the bottom due to the effect of gravity.

2. Loading : It is the process in which chemicals like alum is added to coagulate the floating matter so that impurities become heavier and settle down.

3. Filtration : It is the process in which water is pumped through beds of sand and gravel to remove the suspended matter.

II. Destroying harmful germs :

1. Aeration : This process involves passing of air at high pressure to oxidise all the organic matter and germs which remain dissolved.

2. Chlorination : This process involves disinfection of water by using bleaching powder which releases chlorine and kills germs.

Select the Odd and Out Giving reason

1. Nitrification- Others are related to purification of water.

2. Potable water- Others are different sources of water.

3. Rain water - Others have dissolved impurities

4. Gravitation- Other terms are related to purification of water.

5. Anaemia - It is due to deficiency of iron.

WATER

EXERCISES

A. ANSWER THE FOLLOWING QUESTIONS IN ABOUT TWENTY WORDS.

Q. 1. List five uses of water.

Ans. The uses of water are as follows :

(i) Water is used in cooking.

(ii) Water is used for drinking almost by all living organisms.

(iii) Water is used in industries for various purposes.

(iv) Water is used for cleaning clothes, utensils etc.

(v) Water provides a medium for recreational sports such as

swimming, boating, water skiing etc.

Q.. 2 Why is natural water not considered safe for drinking ?

Ans. The natural water is not considered safe for drinking because

it is contaminated with many impurities that come naturally and also in

the form of different chemicals from industries. Due to these reasons, we

should avoid drinking natural water.

Q.. 3. How is river water purified to m e fit for drinking ?

Ans. River water is purified by chlorination to make it fit for drinking.

In this method chlorine is added to water in specific amount to kill the

germs present in it.

Q.. 4. What is the imp tance of water for humans, animals

and plants ?

Ans. Water is very important humans, animals and plants in the

following ways :

(i) Water regulates body temperature by the process of sweating and

evaporation.

(ii) Water inside the bodies of plants, animals and humans transport

minerals, gases and many products of digestion and carries them

at the place of need in the body.

(iii) Water provides a medium for all biochemical reactions inside our

bodies to take place.

(iv) Water is also needed by plants for their growth and carrying out

photosynthesis.

Q. 5. What is the effect of water on the following and under

what condition :

(a) Sodium metal (b) Iron ?

Ans. (a) Sodium metal reacts with water at room temperature. This

reaction is so fast that hydrogen so produced catches fires.

room temp.

2Na (s) + 2H 2

O (l) → 2NaOH (aq) + H2 (g)

vigorous

Sodium Water Sodium hydroxide Hydrogen

(b) Iron reacts with water only when steam is passed over red-hot

iron. The reaction is reversible.

steam

3Fe (s) + 4H 2

O (g) Fe 3

O 4 (s) + 4H2 (g)

Iron Steam Ferrosoferric Hydrogen

(Red hot) oxide

WATER 2

Q.. 6. raw a labelled diagram of the apparatus used for the

electrolysis of water.

Ans. The labelled diagram used for electrolysis of water is as follows :

water

Electrolysis of water

Acidulated

Electrodes

Glass

tubes

Oxygen

Hydrogen

Q. 7. Why do icecubes float on the surface of water ? Give

reason.

Ans. In ice, water molecules form a three dimensional structure with

large open space between them. Ice occupies more volume than an equal

mass of water. That is why ice is lighter than water and floats over it.

3 TEACHER’S MANUAL :: The World of Science–CHEMISTRY— 8

Q. 8. What is meant by the water of crystallisation ?

Ans. The water molecules present in the crystal lattice of the

substance (per molecule of the substance) is called its water of

crystallisation. Different salts contain different number of water molecules

as the water of crystallisation.

Q. 9. How will you find out if the given sample of water is

soft or hard ? How is hard water softened ?

Ans. Hard water does not form lather with the soap solution. Hard

water can be softened by the following methods :

(i) By boiling : By this method, temporary hardness can be

removed.

(ii) By adding washing soda : Calcium and magnesium ions

present in hard water react with sodium carbonate to produce insoluble

carbonates. These insoluble substances are removed by filtration.

(iii) Ion-exchange resin method : Hardness of water can also

be removed by passing hard water through a column packed with

ion-exchange resins. The ion-exhange resins exchange calcium and

magnesium ions (Ca 2+ and Mg 2+) present in hard water for sodium and

potassium ion (Na + and K + ).

Ion-exchange method can remove both the types of hardness of

water.

Q.. 10. Give one example of hydrated sal

Ans. Sodium carbonate decahydrate or washing soda

(Na 2

CO 3

.10H2

O) is an example of hydrated salt.

Q.. 11. Wha are colloids ? Mention two characteristics of

colloids.

Ans. A homogeneous-looking heterogeneous mixture in which

particles having size between 1 and 100 nm* are dispersed in a

continuous medium is called a colloid. Colloids are commonly called

sols.

Milk, Office paste (gum), Blood, Toothpaste, Jelly, etc., are some

examples of colloids.

The two characteristics of colloids are as follows :

(i) Colloid pa ticles are large enough to cause scattering of light.

(ii) The constituents of a colloid cannot be separated by simple

physical methods like filtration.

Q. 12. How is the temporary hardness of water removed ?

Ans. Temporary hardness of water can easily be removed by boiling.

On boiling, calcium and magnesium bicarbonates decompose to give the

precipitate of calcium and magnesium carbonates. The precipitates are

removed by filtration.

boiling

Ca(HCO 3

) 2 → CaCO 3 (s) + H2

O (l) + CO2 (g)

Calcium bicarbonate Calcium cabonate Water Carbon

(in temporary precipitate (can be dioxide

hard water) removed by filtration)

boiling

Mg(HCO 3

) 2 → MgCO 3 (s) + H2

O (l) + CO2 (g)

Magnesium bicarbonate Magnesium carbonate Water

(in temporary precipitate

hard water) (removed by filtration)

Q. 13. Why can’t we use hardwater for washing our clothes ?

Ans. Soap is the sodium salt of stearic acid (C17

H35

COO Na, sodium

stearate). When soap is added to hard water, it reacts with the calcium

and magnesium ions present in hard water to form insoluble calcium or

magnesium stearate (white and curdy precipitate).

WATER 4

*nm is the symbol for nanometre : 1 nm = 10–9 m

CaCl 2 + 2C17

H35

COONa → 2NaCl + C17

H35

COO) 2

Ca

Calcium chloride Sodium stearate Sodium chloride Calcium stearate

MgCl 2 + 2C17

H35

COONa → 2NaCl + (C17

H35

COO) 2

Mg

Magnesium Sodium stearate Sodium chloride Magnesium

chloride stearate

As a result, soap will not produce lather with water until all the

calcium and magnesium ions have precipitated. This is the reason why

hard water cannot be used for washing (laundry) purposes.

Q. 14. How does water react with the following ?

(a) K 2

O (b) CaO (c) CO 2

(d) Fe (e) Zn

Write balanced chemical equation for the reactions.

Ans.

(a) K 2

O (s) + H2

O (l) → 2KOH (aq)

Potassium oxide Water Potassium hydroxide

(b) CaO (s) + H2

O (l) → Ca(OH) 2 (aq)

Calcium oxide Water Calcium hydroxide

(Lime) (Limewater)

(c) CO 2 (g) + H2

O (l) → H 2

CO 3 (aq)

Carbon dioxide Water Carbonic acid

5 TEACHER’S MANUAL :: The World of Science–CHEMISTRY— 8

steam

(d) 3Fe (s) + 4H2

O (g) Fe 3

O 4 (s) + 4H2 (g)

Iron Steam Ferrosoferric Hydrogen

(Red hot) oxide

boiling

(e) Zn (s) + H 2

O (l) → ZnO (s) + H 2 (g)

Zinc Boiling Zinc oxide Hydrogen

powder water

Q. 15. Describe the characteristic properties of colloids.

Ans. Some characteristic properties of a colloid are described below :

l Heterogeneous nature. A colloid

(or sol) is heterogeneous in nature. The

particles in a colloid can be seen only

with a powerful microscope.

l Particle size. The size of particles in a

colloid lies between 10 –9 m and 10 –7 m.

l Separation of colloidal particles

by filtration. Colloidal particles

can easily pass through the pores of a filter paper. Therefore,

colloidal particles cannot be separated by ordinary filtration.

Particles in a colloid

l Brownian movement of colloidal

particles. The zig-zag motion of particles

in a colloid is called Brownian motion.

l Light scattering by colloidal particles.

Colloids scatter a light beam passing

through them. The scattering of light by

the particles in a colloid is called tyndall

effect. The Tyndall effect is shown in the figure.

Tyndall effect shown by colloid

Brownian motion

of colloidal

B. TICK (4) THE CORRECT CHOICE.

1. The volume ratio of hydrogen and oxygen in water is

(a) 2 : 1 (b) 1 : 2

(c) 1 : 8 (d) 8 : 1

2. How much of the Earth’s surface is covered with water ?

(a) about two-third (b) about three-fourth

(c) about one-fourth (d) about one-third

3. Pure water is an/a

(a) acidic oxide (b) amphoteric oxide

(c) basic oxide (d) none of these

4. Water is used as a coolant in an automobile because of its

(a) high density (b) high specific heat

(c) high boiling point (d) high heating capacity

5. Temporary hardness is due to the presence of

(a) calcium bicarbonate (b) calcium chloride

(c) magnesium chloride (d) magnesium sulphate

6. Both temporary and permanent types of hardness can be

removed by using

(a) washing soda (b) lime

(c) ion-exchange resin (d) hydrochloric acid

WATER 6

7. The loss of dissolved oxygen from water in water bodies is called

(a) photosynthesi

(c) de xygenation

(b) xygenation

(d) eutrophicatio

Ans. 1. (a) 2. (b) 3. (b) 4. (b) 5. (a) 6. (c) 7. (d)

7 TEACHER’S MANUAL :: The World of Science–CHEMISTRY— 8

C. FILL IN THE BLANK SPACES IN THE FOLLOWING STATEMENTS.

1. Water is an essential component of all living beings.

2. Seeds need water for germination.

3. Pure water boils at 100 °C under normal pressure.

4. Water helps in transporting vitamins and minerals from one

place to another.

5. When water is electrolysed, the gas produced at cathode is

hydrogen.

6. Density of water is maximum at 4 °C.

7. Ice at 0 °C is cooler than water at the same temperature.

8. Permanent hardness of water can be removed by adding

washing soda to it.

9. The major source of water pollution in our cities is sewage.

10. Chlorination is the process of killing bacteria present in water

by adding chlorine to it.

D. WRITE T FOR TRUE AND F FOR FALSE STATEMENT.

1. Boiling point of water is the highest of all liquids.

2. Pure water is a good conductor of electricity.

3. Water is denser than ice.

4. High specific heat of water makes it suitable for use in desert

coolers.

5. Ion-exchange method is used for removing permanent hardness

only.

6. Hard water should not be used for drinking.

7. Permanent hard water contains calcium or magnesium

bicarbonate.

8. Supersaturated solution is less concentrated than an unsaturated

solution.

Ans. 1. False 2. False 3. True 4. True 5. False

6. True 7. False 8. False

---

Water, a remarkable substance composed of hydrogen and oxygen, exists in three states: gaseous, liquid, and solid. Here are some key points:

1. Water Cycle:

• The water cycle involves the circulation of water through processes like evaporation and condensation.

• Water evaporates from oceans, rivers, lakes, wells, and soil due to sunlight and heat.

• Transpiration occurs when plants release water vapor from their leaves.

• Condensation converts water vapor back into liquid form, leading to cloud formation.

• Precipitation occurs when water droplets in clouds become heavy and fall as rain, snow, or sleet1.

1. Three States of Water:

• Liquid State: The water we use daily is in the liquid state.

• Gaseous State: Water evaporates to form vapor when heated.

• Solid State: Water turns into ice when cooled1.

1. Availability and Interconvertibility:

• Water’s easy interconvertibility between states ensures its availability across the Earth.

• Evaporation from oceans, rivers, and lakes replenishes water vapor in the atmosphere1.

1. Importance:

• Water is essential for life, dissolving many other substances.

• It sustains ecosystems, supports agriculture, and provides drinking water.

Remember, understanding water’s properties and cycles helps us appreciate its significance in our lives!