6 WATER
I. Tick (3) the most appropriate answer.
1. River water is a
(a) mixture
(b) solution
(c) compound (d) element
2. Potable water is
(a) transparent
(b) transparent and colourless
(c) transparent, colourless and has taste
(d) transparent, colourless, has taste and is free from germs
3. The purest form of natural water is
(a) springwater (b) rainwater (c) wellwater (d) seawater
4. Water used for chemical reactions in the laboratory is purified by
(a) filtration (b) evaporation (c) distillation (d) sedimentation
5. Potable water is obtained from river water by the process of
(a) sedimentation only
(b) sedimentation and filtration only
(c) sedimentation, filtration and chlorination
(d) none of these
6. The product obtained when a solid substance dissolves in a liquid, is
called a
(a) solvent
(b) solute
(c) solution (d) saturated solution
7. Polluted water can cause
(a) malaria
(b) diarrhoea (c) cancer
(d) heart diseases
8. The percentage of water in the human body is nearly
(a) 20%
(b) 60%
(c) 70%
(d) 85%
9. A simple and safe household method for purifying water is
(a) boiling and filtering it
(b) treating it with chemicals
(c) sedimentation and decantation (d) filtering it
10. Which method is not used for supplying drinking water in towns and
cities?
(a) Boiling (b) Decantation (c) Sedimentation (d) Filtration
11. Which of the following is true?
(a) A part of the rainwater runs off to the rivers and merges with the
sea.
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(b) When there is a good rain, the seawater overflows into the
rivers.
(c) Groundwater is not affected by rainwater.
(d) The ice in the high mountains never melts.
12. By which of the following methods is a solute obtained from a
solution?
(a) Sedimentation (b) Decantation (c) Filtration (d) Evaporation
13. How are the germs in water killed?
(a) By sedimentation
(b) By filteration
(c) By boiling
(d) By treating with alum
14. Which of the following is a water-borne disease?
(a) Diphtheria (b) Tetanus
(c) Hepatitis
(d) Malaria
Ans. 1. (a) 2. (d) 3. (b) 4. (c) 5. (c) 6. (c) 7. (b) 8. (c) 9. (a) 10. (a)
11. (a) 12. (d) 13. (c) 14. (c).
II. [A] Fill in the blank spaces by choosing the correct words from the
following list :
List: clay, common salt, people, consumption, cholera, hydroelectric,
leaves, insipid, saturated, pit latrines, water vapour.
1. Water is essential for running ........................ power plants.
2. Cloud or mist is an example of condensed ................... .
3. River water contains ................. and fine particles of sand.
4. Sea water contains 24g of ............... in every litre of water.
5. Plants release water vapour through their ..................... .
6. A sample of water which is fit for human ........................ is called
potable water.
7. Distilled water has a flat and ........................ taste.
8. A solution which cannot dissolve more of the solute is called a
........................ solution.
9. Polluted water can cause diseases like dysentery and
........................ .
10. Water pollution due to defecation by ........................ in the open
can be controlled by using ........................ .
Ans. 1. hydroelectric 2. water vapour 3. clay 4. common salt 5. leaves 6.
consumption 7. insipid 8. saturated 9. cholera 10. people, pit latrines
[B] 1. .................. of the earth’s surface is covered by water (Half/Three-fourth).
2. ........................ is too salty to be used directly (Sea water/Rain water).
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3. ........................ is followed by ........................ in water-supply units
(Sedimentation/Filtration).
4. A solution is a ........................ mixture (heterogeneous/homogeneous).
Ans. 1. Three-fourths 2. Sea water 3. Sedimentation, filtration
4. homogeneous
III. Fill in the blanks:
1. Water is one of the basic constituents of all ........................ .
2. Water at room temperature exists as a ........................ .
3. Rainwater collects on the earth in the form of ........................ and
........................ .
4. The constant movement of water from the ........................ to
........................ and back to ........................ is known as water cycle.
5. Water dissolves many substances; therefore, it is a ................ .
6. Distilled water is the ........................ water.
7. The water suitable for drinking is called ........................ water.
8. ........................ speeds up sedimentation.
9. Sand and dust are ........................ impurities in water.
10. Seawater has high concentration of ........................ .
11. Evaporation of rainwater leaves ........................ residue.
12. A ........................ is a uniform mixture of a solute and a solvent.
13. Ice, water and steam have different physical states but are chemically
........................ .
14. Boiling kills most of the ........................ in water.
15. The elements present in the molecules of water are ........................ and
........................ .
Ans. 1. plants and animals 2. liquid 3. surface water, groundwater 4. earth, air,
earth 5. universal solvent 6. purest form of 7. potable 8. alum
9. suspended 10. common salt 11. no 12. solution 13. same
14. microorganisms 15. hydrogen, oxygen
IV. Statements given below are incorrect. Write the correct statements.
1. Distilled water is fit for consumption by human beings.
Ans. Distilled water is not fit for consumption by human beings because it
does not contain any mineral salts in it.
2. The amount of solute which dissolves in 1000 g of water to form a
saturated solution is called its solubility.
Ans. The amount of solute which dissolves in 100 gm of water to form a
saturated solution is called its solubility.
3. The latent heat of ice is 540 calories per gram.
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Ans. The latent heat of ice is 80 calories/gram.
4. Potable water is obtained from sea water by the process of
evaporation.
Ans. Potable water is obtained from some natural lakes or rivers which is
then subjected to various kinds of treatment so that it is freed from
suspended impurities and harmful microorganisms.
V.
State whether the following statements are true or false :
1. Pure water is a colourless liquid having a fine taste.
2. All living beings contain 10% to 20% of water.
3. Excessive use of fertilisers does not cause water pollution.
4. Seawater contains 24 g of common salt per litre.
5. Dissolved gases in natural water are necessary for respiration.
6. Use of pesticides in excess causes water pollution.
7. Cholera is a disease caused due to water pollution.
8. Chlorine gas is added to clear water, so as to kill germs and bacteria.
9. River water is the most impure form of water.
10. Nearly one-fourth of the earth’s surface is covered with water.
11. Water boils at 100°C.
12. Water can dissolve gases.
13. The three states of water are not interconvertible.
14. Pure water is tasteless.
15. Water cycle is a continuous process.
16. The amount of water on the earth is decreasing due to evaporation.
17. A saturated solution can take up any amount of the solvent.
18. Pollutants are produced by natural processes.
19. One should never drink polluted water.
20. Water has maximum density of 1 gm/cm3 at 4°C.
21. Water is an element.
22. Tap water does not contain dissolved impurities.
23. Distillation is a good method for purifying water for town-supply.
Ans. 1. F 2. F 3. F 4. T 5. T 6. T 7. T 8. T 9. T 10. F 11. T 12. T 13. F
14. T 15. T 16. T 17. F 18. F 19. T 20. T 21. F 22. F 23. F
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VI.
(a) Match the statements in Column A with Column B.
Column A
Column B
1. Wellwater is generally
a. rainwater
2. Purest form of natural water
b. pond water
3. Impurest form of water
c. water pollution
4. The process by which suspended
d. sedimentation
impurities in river water settle down
5. The gas used for killing germs in city water
e. cholera
6. Disease caused by water pollution
f. chlorine
7. Washing down of excessive pesticides
g. underground water
and fertilisers into river water
Ans. 1. (g) 2. (a) 3. (b) 4. (d) 5. (f) 6. (e) 7. (c).
(b)
1.
2.
3.
4.
5.
Column A
Distillation
Underground water
Sedimentation
Solution in water
Solute
a.
b.
c.
d.
e.
Column B
can be obtained by digging wells
pure water
substances that dissolve in a solvent
settling down of heavier particles
aqueous solution
Ans. 1. (b) 2. (a) 3. (d) 4. (e) 5. (c).
(c)
Column A
1. Solute
2. Solvent
3. Stirring and heating
a.
b.
c.
4. Saturated solution
d.
5. Water
e.
Column B
No more solute
Universal solvent
Recovered from a solution by
evaporation
Recovered from a solution by
distillation.
Help to dissolve a solute
Ans. 1. (c) 2. (d) 3. (e) 4. (a) 5. (b).
VII. Name:
1. Two chemicals used to destroy the germs present in water.
Ans. Chemicals like chlorine and ozone are used to destroy germs present
in water.
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2. A process used to separate common salt from seawater.
Ans. Evaporation is the process used to recover common salt from
seawater.
3. Two diseases which spread through impure water.
Ans. Diarrhoea and gastroenteritis are common water-borne diseases.
4. Two underground sources of water used for drinking purposes.
Ans. There are mainly two types of underground water which are used for
drinking purposes: 1. Wellwater
2. Springwater.
5. A chemical to test water.
Ans. Anhydrous white copper sulphate is used to test water.
6. Four surface water sources.
Ans. Rivers, lakes, ponds and streams are the sources of surface water.
VIII. Find out the odd one out. Give a reason for your answer.
1. Diarrhoea, jaundice, malaria and typhoid.
Ans. Diarrhoea, jaundice, and typhoid are the diseases spread by polluted
water while malaria is spread by mosquito.
2. Springwater, potable water, well water and river water.
Ans. Springwater, wellwater and river water are naturally occurring sources
of water while the water that is fit for human consumption rom any
source is known as potable water.
3. Ponds, lakes, seas and rivers.
Ans. Ponds, lakes and rivers do not have salty water while seawater is salty
in nature.
4. Sedimentation, decantation, defection and filtration.
Ans. Sedimentation, decantation and filtration are the methods used to
purify water while due to defecation, water pollution is increasing.
5. Sewage water, fertilisers in water, chlorine gas and waste water from
factories.
Ans. Sewage water, fertilisers in water, and waste water from factories all
act as water pollutants while chlorine gas is used to kill harmful
microorganisms in water.
6. Human waste, oil, waste from factories, chlorine gas, detergents,
untreated sewage, fertilisers.
Ans. Human waste, oil, waste from factories, detergents, untreated sewage
and fertilisers cause water pollution while chlorine is used to kill
microorganisms in polluted water.
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IX.
Answer the following questions.
1. State five important uses of water.
Ans. Uses of water:
(i) Water is vital for life. Plants and animals cannot survive without
water. About 70% of the human body is made of water.
(ii) Water is essential for the growth of plants.
(iii) Water is used for power production, i.e., for running
hydroelectric and thermoelectric power plants.
(iv) Water is used for our daily necessities such as bathing, cooking,
washing clothes, etc.
(v) In most of the countries, water is used for bulk transport of
goods to other countries, through the sea.
2. Rainwater is considered the purest form of natural water. Explain the
statement. Name the gases which dissolve in rain water before it
reaches the ground. What harm can be caused by these gases?
Ans. Rainwater is considered the purest form of natural water because it is
formed by the distillation of surface water. Although rainwater is the
purest form of natural water, it contains some gaseous impurities like
oxygen, nitrogen, carbon dioxide and sulphur dioxide. Carbon dioxide
and sulphur dioxide react with water and form carbonic acid and
sulphuric acid respectively. They damage crops, forests, buildings and
the environment.
3. List (i) three dissolved impurities, and (ii) three suspended impurities
present in river water.
Ans. Three dissolved impurities in river water are:
(i) Salts of calcium
(ii) Hydrogen sulphide
(iii) Carbon dioxide.
Three suspended impurities in river water are:
(i) Sand
(ii) Clay
(iii) Organic matters such as wood and dead animals.
4. Name the main salt present and its percentage in seawater.
Ans. Common salt (sodium chloride) is the main salt present in seawater.
The percentage of common salt in seawater is 2.4%.
5. Describe an experiment to prove that plants give out a large amount
of water vapour.
Ans Experiment to show that plants release water vapour through
their leaves. Take two beakers and pour 50 cc water and 10 cc of oil
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6.
Ans.
7.
Ans.
8.
Ans.
9.
Ans
in each. Put a freshly cut twig of a plant in one beaker. Place both the
beakers in the sun and cover them with a dry bell jars. After two hours
we notice that the bell jar having freshly cut twig has tiny droplets of
water clinging to its sides. It is because the leaves of the twig give out
water vapour in air.
Write a short note on water cycle.
Water Cycle. The constant flow of water on the earth between the
atmosphere, land and ocean is called water cycle.
Due to burning of fuels, evaporation of water from water bodies,
transpiration and breathing, enormous amount of water vapour is
released into the air. It rises up and condenses to form clouds. Clouds
cause rain. Rainwater flows over the surface of the earth. Some of it
is collected as underground water and some of it is collected in
springs, streams and rivers. Thus a natural balance of water is
maintained.
State five qualities of drinking water.
Qualities of potable drinking water
(i) It must be clear and free from suspended impurities.
(ii) It must be odourless.
(iii) It must be colourless.
(iv) It must be free from disease causing microorganisms.
(v) It must contain small amounts of mineral salts of sodium and
calcium that give it a special taste.
State two ways by which a small amount of clear water can be made
safe for drinking.
The two ways by which a small amount of clear water can be made
safe for drinking are by boiling and chemical treatment with chorine.
What do you understand by the following terms?
(i) Solute
(ii) Solvent
(iii) Solution
(iv) Saturated solution. (v) Solubility of solute.
(i) Solute. A substance which dissolves in a liquid is called a solute.
For example, common salt, sugar, etc.
(ii) Solvent. A liquid which dissolves other substances in it, is called
a solvent. For example, water is a solvent that dissolves sugar in
it.
(iii) Solution. When a solute dissolves in a solvent, then the resulting
mixture is called a solution. For example, when sugar dissolves
in water, then the product so obtained is called a sugar solution.
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10.
Ans.
11.
Ans.
(iv) Saturated solution. A solution which cannot dissolve any more
of a solute at a given temperature is called a saturated solution at
that temperature.
(v) Solubility of solute. The maximum amount of solute which can
be dissolved in 100 gm of water at a given temperature so as to
form a saturated solution is called solubility of the solute at that
temperature.
How does pollution of water take place:
(i) due to daily activities of men and animals
(ii) industries and agricultural operations?
(i) Water pollution due to daily activities of men and animals.
In rural areas, people and animals defecate anywhere and
everywhere. When it rains, this excreta is carried to streams and
rivers and causes water pollution.
In many places, this excreta is directly dumped into the rivers.
This pollutes the water to a greater extent.
(ii) Water pollution due to industrial as well as agricultural
operations. Water discharged from the industries contains
metallic salts of mercury, lead, cadmium, arsenic, etc. These
salts are highly poisonous and cause water pollution. If this water
is utilised by plants and animals, it causes serious diseases.
During agricultural activities, farmers use artificial fertilisers,
pesticides and insectides. These are washed down to the rivers
and water bodies and cause pollution of potable water.
How can the pollution caused by the activities of men and industrial
as well as agricultural operations be minimised?
Water pollution caused by the daily activities of men and animals can
be minimised by the following steps—
(i) In villages, people should be encouraged to use pit latrines.
(ii) Cowduing should be converted into biogas in biogas plants.
(iii) Sewage should be treated properly to remove all the harmful
substances before discharging into rivers.
(iv) Waste water of factories should be treated with chemical
substances to remove harmful metallic salts.
(v) By using large amount of natural manure and natural pesticides.
(vi) By using drip irrigation so that harmful bacteria do not percolate
down.
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12. State the sources of water.
Ans. The sources of water are oceans, rivers, lakes, streams, ponds, springs
and rainwater. Dams, wells, tubewells and canals are some man-made
sources of water.
13. How is interchangeability of water beneficial to us?
Ans. Interchangeability of water is beneficial to us because due to this the
phenomenon of water cycle comes into existence which makes life
possible on earth.
14. How is drinking water provided in towns and cities?
Ans. Drinking water provided in towns and cities must be free from
suspended impurities and harmful microorganisms. This is done in
four stages.
(i) Sedimentation. Water is pumped into a series of settling tanks
and allowed to stay still for a day. Very heavy particles of sand,
clay, etc. settle at the bottom of the settling tank.
(ii) Addition of chemicals and further settling. Now water is
mixed with alum and lime when fine particles of sand and clay
form a bulky sticky precipitate. They settle down and the clear
water is pumped out.
(iii) Filtration through sand and gravel. It completely removes
suspended impurities.
(iv) Chlorination of water. Clear water is chlorinated so as to
remove harmful bacteria present in it. This clear water is
distributed to towns and cities.
15. Name some substances which pollute water.
Ans. Sewage from town and cities, wastes from factories and industries,
chemicals such as fertilisers and pesticides are the major causes of
water pollution.
16. Why is water called a universal solvent?
Ans. Water is called a universal solvent because of its ability to dissolve
most of the compounds.
Water can dissolve in it all kinds of solids, liquids or gases. For
example, sugar, common salt, etc., are soluble substances.
17. What are the states in which water is present in nature?
Ans. In nature, water exists in the three states:
Solid state as ice.
Liquid state as liquid water.
Gaseous state as water vapour.
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18. Name two biochemical reactions in which water takes part.
Ans. Photosynthesis and germination are the biochemical reactions in
which water is used.
19. Which water contains the highest concentration of salt in it?
Ans. Seawater contains the highest concentration of salt in it.
20. Why should drinking water be purified before use?
Ans. Water should be purified before its consumption because polluted
water contain many impurities which might be harmful for human
beings or animals.
21. What is mineral water?
Ans. Mineral water is colourless and odourless. It contains some minerals
that are necessary for our body. It also contains some dissolved gases
to add taste.
22. What are the three methods of removing germs from natural water?
Explain.
Ans. The three methods of removing germs from natural water are.
(i) By exposure to air and sunlight. It is the oldest method to kill
germs in water. Air and sunlight have the effect of burning germs
to death.
(ii) By boiling. It is the best method to kill all the germs for small
quantities of water.
(iii) By chemical treatment. Chemicals like chlorine and ozone are
used to kill bacteria.
23. How is water in a swimming pool kept free from infectious germs?
Ans. Swimming pools are usually chlorinated to keep the water free from
infectious germs.
24. What is the purpose of adding bleaching powder to water supplied to
the town?
Ans. Bleaching powder releases chlorine when added to water. Chlorine
kills harmful microorganisms present in water.
25. Why is spring water pure enough for drinking but unsuitable for
laboratory use?
Ans. Springwater is usually clear and free from suspended particles
because it is naturally filtered through several layers of gravel and
sand. Springwater is unsuitable for laboratory use because it contains
many dissolved minerals.
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