Living Science Chemistry Class 7 ICSE Solutions Chapter 6 Water

ICSE Living Science Chemistry for Class 7 Solutions Chapter 6 Water

Check Your Progress

Answer these questions.
Question 1.
Name some sources of surface water.
Answer:
Oceans, seas, rivers, lakes and streams.

Question 2.
Name some dissolved impurities in water.
Answer:
Salts of calcium, magnesium, sodium, etc.

Question 3.
What is loading ?
Answer:
Loading is a method of removing suspended impurities in water in which water is treated with potash alum to increase the rate of sedimentation in a loading tank. Mixing with potash alum, fine suspended particles become heavy and they settle down at the bottom of the tank.

Question 4.
Which chemical is used for chlorination of water ?
Answer:
Bleaching powder which releases chlorine in water.

Question 5.
What is ozonization ?
Answer:
The process of using ozone gas for disinfection of drinking water is called ozonization. It does not affect the taste or odour of water.

A. Tick the most appropriate answer.

Question 1.
Rainwater that seeps through the soil and collects above non-porous rocks is called
Answer:
a. groundwater

Question 2.
Chlorination of water is done by using
a. bleaching powder
b. bromine gas
c. sulphur dioxide gas
d. hydrogen gas
Answer:
a. bleaching powder

Question 3.
The metal that reacts with water to liberate hydrogen gas is
a. sodium
b. gold
c. mercury
d. silver
Answer:
a. sodium

Question 4.
Water reacts with carbon dioxide to form
a. carbonic acid
b. sulphuric acid
c. hydrochloric acid
d. none of these
Answer:
a. carbonic acid

Question 5.
Temporary hardness of water can be removed by
a. freezing
b. boiling
c. melting
d. all of these
Answer:
b. boiling

Question 6.
The presence of bicarbonates of calcium and magnesium causes
a. temporary hardness of water
b. permanent hardness of water
c. temporary and permanent hardness of water
d. none of these
Answer:
a. temporary hardness of water

Question 7.
Which of the following does not cause hardness of water? *
a. Calcium bicarbonate
b. Sodium bicarbonate
c. Magnesium chloride
d. Calcium sulphate
Answer:
b. Sodium bicarbonate

Question 8.
The content of dissolved oxygen decreases with increase of
a. pressure
b. temperature
c. moisture
d. all of these
Answer:
b. temperature

B. Fill in the blanks.

Question 1.
The continuous circulation of water from the earth to the atmosphere and back to the earth is known as
Answer:
The continuous circulation of water from the earth to the atmosphere and back to the earth is known as water cycle or hydrological cycle.

Question 2.
The presence of impurities in water the boiling point nd the freezing point of water.
Answer:
The presence of impurities in water raises the boiling point and lowers the freezing point of water.

Question 3.
Pure water is a conductor of heat and electricity.
Answer:
Pure water is a bad conductor of heat and electricity.

Question 4.
The density of water is maximum at
Answer:
The density of water is maximum at 4°C

Question 5.
The expansion of water helps aquatic animals to survive in frozen ponds or lakes in winter.
Answer:
The anomalous expansion of water helps aquatic animals to survive in frozen ponds or lakes in winter.

Question 6.
Temporary hardness of water can be removed by which decomposes the soluble present in the water.
Answer:
Temporary hardness of water can be removed by boiling which decomposes the soluble bicarbonates of calcium and magnesium present in the water.

C. State if the following statements are true or false. Correct the statement if it is false.

Question 1.
Increase in pressure decreases the boiling point of water.
Answer:
False. Increase in pressure increases the boiling point of water.

Question 2.
Pure water has its maximum density at 0°C.
Answer:
False. Pure water has its maximum density at 4°C.

Question 3.
Ice at 0°C has a lower density than water at 4°C and hence it floats on water.
Answer:
True

Question 4.
Chemically pure water is called distilled water.
Answer:
True

Question 5.
Alcohol is immiscible in water whereas petrol is miscible in water.
Answer:
False. Alcohol is miscible water whereas petrol is immiscible in water.

Question 6.
Aquatic animals like fish use oxygen from the atmosphere for respiration.
Answer:
False. Aquatic animals like fish use oxygen from the water for respiration.

D. Name the following.

Question 1.
The continuous circulation of water in nature
Answer:
Water cycle or hydrological cycle

Question 2.
Water found in the oceans, seas, rivers, etc.
Answer:
Surface water

Question 3.
Water that is fit for drinking
Answer:
Potable water

Question 4.
The chemical added to kill germs during purification of water
Answer:
Chlorine (bleaching powder)

Question 5.
The substance formed when a metal oxide is dissolved in water
Answer:
Metal hydroxide

Question 6.
The acid formed when sulphur trioxide reacts with water
Answer:
Sulphuric acid (H₂SO₄)

E. Match the following.

1. Dissolved salts of calcium and magnesium	a. increases the rate of sedimentation of suspended impurities
2. Potash alum	b. hardness of water
3. Bleaching powder	c. 4°C
4. Maximum density of water	d. lathers well with soap
5. Soft water	e. disinfectant

Answer:

1. Dissolved salts of calcium and magnesium	b. hardness of water
2. Potash alum	a. increases the rate of sedimentation of suspended impurities
3. Bleaching powder	c. 4°C
4. Maximum density of water	d. lathers well with soap
5. Soft water	e. disinfectant

F. Complete and balance the ; rollowing equations.

1. K + 2H₂O →
Answer:’

2. Na + H₂O →
Answer:

3. Na₂O + H₂O →
Answer”:

4. MgO + H₂O →
Answer:

5. SO₃ + H₂O →
Answer:

6. Ca(HCO₃)₂ heat
Answer:

7. CaCl₂ + Na₂CO₃ →
Answer:

8. MgSO₄ + Na₂CO₃ →
Answer:

G. Answer the following in short.

Question 1.
Define water cycle. Give its other name.
Answer:
The continuous circulation of water from the earth to the atmosphere and back to the earth is called as the water cycle. This cycle involves evaporation, condensation and precipitation. The water cycle is also called as hydrological cycle.

Question 2.
Why is sea water not fit for drinking ?
Answer:
Sea water is not fit for drinking as it contains large amounts of dissolved salts.

Question 3.
What is groundwater ? State its two main sources.
Answer:
Some amount of water from the sources of surface water
and rain seeps into the ground and collects above the non- porous rocks to form groundwater. Sources of groundwater include springs and wells.

Question 4.
Why is distilled water not fit for drinking ?
Answer:
Distilled water is not good for drinking as it tastes flat and lacks dissolved salt and minerals required for our body.

Question 5.
Why is potash alum added to the loading tank ?
Answer:
Mixing with potash alum, fine suspended particles become heavy and settle down at the bottom of the loading tank. It increases the rate of sedimentation.

Question 6.
What is the maximum density of water and at what temperature does it occur ?
Answer:
Density of water is lg/cm³ at 4°C.

Question 7.
Why does ice float on water ?
Answer:
Density of ice at 0°C is less than the density of water. Therefore, ice floats on the surface of water.

Question 8.
What is the importance of dissolved carbon dioxide and oxygen in water ?
Answer:
Aquatic plants use dissolved Carbon dioxide in water for photosynthesis and aquatic animals use dissolved oxygen in water for respiration.

Question 9.
Why is water called the universal solvent ?
Answer:
Water has the ability to dissolve many substances to form aqueous solutions. Therefore, it is described as the universal solvent.

Question 10.
What causes permanent hardness of water ?
Answer:
Permanent hardness of water is due to the presence of dissolved chlorides and sulphates of calcium and magnesium.

H. Answer the following in detail.

Question 1.
Mention the ways by which
a. suspended impurities can be removed from drinking water.
Answer:
The suspended impurities from water is removed by the following methods,

1. Sedimentation – Water is fed into the sedimentation tanks. The large solid particles are allowed to settle down as sediment.
2. Loading – In the loading tank, water is treated with potash alum to increase the rate of sedimentation. Fine suspended particles become heavy after mixing with potash alum and settle down at the bottom of the tank.
3. Filteration – Following loading, water is passed through Alteration tank. The filteration tank has filter beds that contain layers of fine sand, gravel and charcoal. The filter beds remove all suspended impurities from the water.

b. germs present in drinking water are destroyed.
Answer:
Germs present in drinking water can be destroyed by –

1. Chlorination – The filtered water is treated by bleaching powder which releases chlorine when dissolved in water and destroys harmful microorganisms in it. The addition of chlorine to drinking water is called chlorination.
2. UV radiation – These destroy harmful microorganisms from water. It does not affect the taste and odour of the water.
3. Ozonization – Ozone gas is also used to disinfect drinking water. It does not affect the taste and odour of water.

Question 2.
Describe experimentally with the help of a labelled diagram, the determination of the boiling point of pure water.
Answer:
Aini: To determine the boiling point of water Materials Required : A beaker, a wire gauze, a tripod stand, a Bunsen burner, a thermonjeter, distilled water.

Procedure :

1. Pour some distilled water in a beaker.
2. Set up the apparatus as shown in the figure below.
3. The bulb of the thermometer should just dip in the water.
4. Note the temperature on the thermometer. Heat the water till .it boils.

Observation : As the water boils, the temperature reaches 100°C and becomes constant.
Conclusion : The boiling point of pure water is 100°C.

Question 3.
Explain ‘anomalous expansion of water’ and its significance.
Answer:
Generally, substances contract when they are cooled. However, water shows an anomalous or unusual behaviour. It contracts until the temperature reaches 4°C and it expands when it is cooled below 4°C. This is called anomalous expansion of water. The density of water is maximum at 4°C. As water is cooled below 4°C, it expands instead of contracting. Hence, its volume increases which means the density decreases. Hence, density of ice at 0°C is less than the density of water. Therefore, ice floats on the surface of water.

The anomalous expansion of water is helpful for aquatic life in frozen ponds and lakes to survive during winters or in cold regions. When the temperature falls to 0°C or below, the water on the surface of the ponds or lakes cools and freezes first to form ice. Ice floats over water. Being a bad conductor of • heat, ice does not allow heat to reach the bottom of the pond or lakes and hence the water at the bottom does not freeze. As a result, aquatic animals survive underneath.

Question 4.
How does washing soda remove hardness of water ? Write associated chemical equations.
Answer:
Washing soda removes permanent hardness of wafer which cannot be removed by boiling. Washing soda (sodium carbonate) reacts with chloride and sulphates of calcium and magnesium present in permanent hard water to form their respective insoluble carbonates. The insoluble carbonates can be filtered out and the water becomes soft.

Question 5.
What is water pollution ? Mention three causes for water pollution.
Answer:
The release of substances into surface water or groundwater which makes the water unfit for use by humans or affects living organisms and their environment is called water pollution. Causes of water pollution –
1. Domestic sewage/wastes – Untreated sewage from cities and  towns contains disease – causing microorganisms. When it is discharged into water bodies, the water gets contaminated.

2. Chemical pollution – Water gets contaminated by harmful . chemicals Industrial waste contains poisonous chemicals which affects the quality of water when discharged into it carelessly. Use of pesticides and fertilizers in agricultural fields pollute the groundwater. Accidental oil -spills in the oceans and seas damage marine life too.

3. Thermal pollution – Thermal power plants and oil refineries use large quantities of water for cooling purposes. They release this water at a very high temperature into nearly water bodies (rivers). This leads to thermal pollution of water. Heat decreases the capacity of water to hold dissolved oxygen. This reduced oxygen content badly affects fish and other aquatic animals.

Question 6.
State any five methods to cheek water pollution.
Answer:
Methods to check water pollution are :

1. The chemical waste released from industries should be treated for removal of toxic substances before it is released into water bodies.
2. Domestic sewage should be treated before it is released into water bodies.
3. Excessive use of insecticides and pesticides should be reduced. Organic farming should be encouraged.
4. Activities like bathing animals and washing clothes in water bodies should be avoided.
5. Things like paint, oil, polishes and chemicals should not be thrown into drains and water bodies.

I. Give reasons for the following.

Question 1.
Water pipes sometimes burst during severe winter.
Answer:
In severe winters, temperature reaches 0°C or below it. This causes water to freeze in water pipes. When water freezes, it expands and occupies larger space than liquid water because of anomalous expansion of water and exerts force on the pipes. As a result, the water pipes burst.

Question 2.
Hard water is unfit for washing purposes.
Answer:
It is because hard water does not form lather easily with soap.

Question 3.
Pressure cooker should be used for cooking
Answer:
As the pressure increases, the boiling point of water also increases. A pressure cooker works on this principle. In a pressure cooker, the pressure is high and therefore the water boils at a temperature above 100°C.

Question 4.
Rainwater harvesting has become important these days.
Answer:
Rainwater harvesting has become important these days to conserve water. Water supply in urban area is getting short against the demand. Rainwater is collected and stored for use when water is scarce. Groundwater is getting depleted. Rainwater harvesting replenishes groundwater. It is also a good substitute to polluted water as rain in the purest form of natural water.

J. Draw the diagram of the following. 
Question 1.
To show the process of purification of water in waterworks
Answer:

Living Science Chemistry Class 7 ICSE Solutions