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Page No 229:

Question 1:

Boiling of water is useful because it
A. kills germs.
B. may soften the water.
C. helps it taste better.
D. makes it colourless and odourless.

(a) A, B and C are correct
(b) B, C, D are correct
(c) A and B are correct.
(d) C and D are correct.

Answer:

(c) A and B are correct.

Boiling of water is useful because it kills germs and softens water.

Page No 229:

Question 2:

Which of the following statements are incorrect?
A. Water boils at 100°C.
B. Water than has been filtered through a muslin cloth is safe to drink.
C. Water must be boiled to kill disease causing germs.
D. Water is chlorinated to improve its taste.
E. Sedimentation and adding of alum helps the particles floating in water to settle down.
F. Water evaporates at all temperatures between 0°C and 100°C.

(a) A, B. D
(b) A, C, E
(c) B, C, F
(d) B, D

Answer:

(d) B, D

Statements B and D are incorrect.
Water that has been filtered through a muslin cloth is not safe to drink. Muslin cloth only filters mud. However, this water does contain invisible impurities.
Water is chlorinated to kill harmful microorganisms present in it.

Page No 229:

Question 3:

Which of the following methods could be used at home to make water safe for drinking?
A. filtration
B. adding chlorine tablets
C. boiling
D. sedimentation

(a) all of them
(b) A and D only
(c) A only
(d) B and C

Answer:

(a) all of them

Filtration, adding chlorine tablets, boiling and sedimentation are methods that can be used at home to make water safe for drinking.



Page No 230:

Question 4:

Which of the following substances can be used to soften a sample of hard water?
(a) soap
(b) washing soda
(c) lime
(d) calcium carbonate

Answer:

(b) washing soda

Hard water can be softened by treating it with washing soda or sodium carbonate.

Page No 230:

Question 5:

Which are the processes by which water vapour is added to the atmosphere?
A. transpiration
B. precipitation
C. condensation
D. evaporation

(a) B and C
(b) B and D
(c) A and D
(d) A and B

Answer:

(c) A and D
Both transpiration and evaporation add water vapour to the atmosphere. Transpiration is a process by which plant loses water through its leaves and evaporation is a process by which water changes into vapour.

Page No 230:

Question 6:

Precipitation is a word which means
(a) water goes from the earth and water bodies into the atmosphere.
(b) water returns to earth in the form of rain, snow and hail.
(c) water vapour turns into clouds.
(d) leaves lose water.

Answer:

(b) water returns to earth in the form of rain, snow and hail.
Precipitation is defined as any form of water (solid or liquid) falling from sky to earth. It includes rain, snow and hail.

Page No 230:

Question 7:

Which of these is a salt that does not cause hardness in water
(a) Na2SO4
(b) Ca(HCO3)2
(c) CaSO4
(d) MgSO4

Answer:

(a) Na2SO4

Hardness in water is most commonly caused by bicarbonates and sulphates of calcium and magnesium.

Page No 230:

Question 8:

Rainwater harvesting involves
A. collection of rain as it falls.
B. storing it in an underground tank.
C. using chemicals to treat the water.
D. feeding the water directly into the pipes of the house.

(a) A and C
(b) A and B
(c) C and D
(d) B and D

Answer:

(b) A and B

Rainwater harvesting is a simple and economical way of preserving rainwater that falls on earth. It involves collection and storage of rainwater in an underground tank. 

Page No 230:

Question 9:

Removal of salt from seawater (desalination) can be carried out by
(a) evaporation of salt water.
(b) cooling salt water.
(c) distillation of salt water.
(d) filtration of salt water.

Answer:

(a) evaporation of salt water
Desalination is a process of removing salt from seawater. It can be carried out by evaporation of salt water. As water evaporates, it leaves behind salt.

Page No 230:

Question 10:

Which alphabet would match 'water'?
figure

Answer:

Alphabet 'c' represents water.
Water is a chemical substance that does not dissolve calcium carbonate. It is necessary for good health.

Page No 230:

Question 11:

In the past, waste water from the kitchens and bathrooms was often sent into the vegetable garden of a house. This was done because
(a) the water was reused to water the garden.
(b) there were no drains.
(c) it saved the gardener a lot of work.
(d) it improved the quality of the vegetables.

Answer:

 (a) the water was reused to water the garden.

The waste water from kitchens and bathrooms can be reused to water the garden. It is one of the best ways to conserve water.



Page No 231:

Question 12:

Which of the following statements are true? Rewrite the false statements after correction.
(a) Water which looks clean and sparkling is suitable for drinking.
(b) soap lathers well in soft water.
(c) sodium sulphate is one of the substances which cause hardness in water.
(d) insoluble salts of calcium cause hardness in water.
(e) Rainwater, as it falls, is always pure.

Answer:

(a) False.
Water that looks clean and sparkling may not be suitable for drinking. It may contain invisible harmful microorganisms in it.

(b) True.

(c) False.
Hardness in water is caused by insoluble salts of calcium and magnesium.

(d) True.

(e) True.

Page No 231:

Question 13:

Pooja lived in an area where the water, which was obtained from a tube-well, was usually hard. One day, there was a water shortage and  she took boiled water from the kettle to wash her face. To her surprise the soap lathered well in that water. Why did this happen?

Answer:

This happened because Pooja used soft water instead of hard water. The hardness of water was removed by boiling. The hardness in water is caused by bicarbonates and sulphates of calcium and magnesium. When water is boiled, soluble bicarbonates decompose into insoluble carbonates, which can be removed by filtering water through a fine cloth. This converts hard water into soft water.

Page No 231:

Question 14:

Name two substances that cause hardness in water. Explain how they may be removed and the water softened, giving equations in each case.

Answer:

Bicarbonates and sulphates of calcium and magnesium are two salts that cause hardness in water.
(a) Removal of hardness of water caused by bicarbonates of calcium and magnesium:
It can be removed by boiling. During boiling, soluble bicarbonates decompose into insoluble carbonates. This insoluble salt can be removed by filtering water through a fine cloth at home.
Calcium bicarbonate → Calcium carbonate + Water + Carbon dioxide
Ca(HCO3)2 ⟶ CaCO3 + H2O + CO2

Magnesium bicarbonate → Magnesium carbonate + Water + Carbon dioxide
Mg(HCO3)2 ⟶ MgCO3 + H2O + CO2

(b) Removal of hardness of water caused by sulphates of calcium and magnesium:
It can be done by treating hard water with washing soda or sodium carbonate. The insoluble carbonates of calcium and magnesium can be filtered out but sodium sulphate is allowed to remain in the solution.
Calcium sulphate + Sodium carbonate → Calcium carbonate + Sodium sulphate
CaSO4 + Na2CO3 ⟶ CaCO3 + Na2SO4

Page No 231:

Question 15:

Why does fur form inside a kettle in which water has been boiled for a long time? Give the chemical names and formulae of this fur? In what way is it a nuisance?

Answer:

Repeated boiling of hard water leads to the deposition of insoluble material called fur or scale.  
The chemical name of this fur is calcium carbonate (CaCO3) or magnesium carbonate (MgCO3).  
It is a nuisance as it may coat the heating element in kettles, making them less efficient. As a result, it takes extra time for water to get boiled in such vessels. Also, this fur or insoluble calcium carbonate is unpleasant and it clogs up hot water pipes and boilers.    

Page No 231:

Question 16:

A forgetful scientist took three equal samples of water and added 2 ml of soap solution to each of them. He measured the height of the lather formed in each case. His results were 2 cm; 3 cm and 4 cm. Fill in his results correctly in the table given. Which sample was the hardest? Which is the softest? Can you explain the difference in the results between tap water and boiled water?
 

Water Sample Height of Lather
distilled water  
tap water  
boiled tap water  

Answer:

Water Sample Height of Lather
Distilled water  4 cm
Tap water  2 cm
Boiled tap water  3 cm
 
Tap water is the hardest and distilled water is the softest.

Difference in results between tap water and boiled tap water:
Tap water contains relatively high amount of calcium and magnesium. This little bit hardness of tap water can be removed by boiling. Hence, when tap water is boiled, the soluble bicarbonates decompose into insoluble carbonates, which can be removed by filtering water through a fine cloth at home. Hence, tap water, due a little hardness, gives little lather, whereas boiled tap water gives comparatively more lather when mixed with soap.

Page No 231:

Question 17:

In a village, the local pond is usually used by villagers for all their requirements of water. Make a list of at least five activities of the villagers which might pollute the pond and make the water unfit to drink.

Answer:

Five activities of the villagers that might pollute the pond are as follows:

  • People take bath in the pond and release organic matter into it. This gets accumulated in the pond.
  • People wash their clothes and utensils and pollute the pond.
  • People living on the side of the pond usually lay their latrines on its banks.
  • Villagers allow their cattle to take bath in the pond, making its water unfit to drink.
  • Fertilisers and pesticides are also sources of pollution. In many farms, chemicals are sprayed on crops to help them grow and prevent from pests. When it rains, the extra fertilisers and pesticides flow into the streams and causes pollution of water bodies.

Page No 231:

Question 18:

How do plants help in recharging the groundwater?

Answer:

Trees and plants in a forest help in the seepage of rainwater into the ground. As rainwater seeps into the ground, some of it binds to the particles of soil or to the roots of plants just below the land surface. This moisture provides plants with the water they need to grow. Water not used by plants moves deeper into the ground. The water moves downward through empty spaces or cracks in the soil, sand or rocks and reaches a layer of rock through which water cannot easily move. The water then fills the empty spaces and cracks above that layer. The top of the water in the soil, sand, or rocks is called the water table and the water that fills the empty spaces and cracks is called groundwater.

Page No 231:

Question 19:

Why do some areas with plenty of rainfall suffer from drought-like conditions during seasons other than the monsoon?

Answer:

Some areas, although they receive plenty of rainfall, suffer from drought-like conditions because of deforestation. Due to absence of trees, soil cannot hold much water. As a result, rainwater runs down quickly and is not absorbed by the ground.

Page No 231:

Question 20:

What is rainwater harvesting? How does it help in solving problems of water scarcity?

Answer:

Rainwater harvesting is a simple and economical way of preserving rainwater that falls on earth. It involves collection of rainwater on the roofs of the buildings and storing it underground for later use.

Rainwater harvesting conserves valuable groundwater. It allows the rainwater to seep into the ground more efficiently by constructing percolation pits and recharge wells. It raises the water level in the wells and borewells that are drying up, thereby solving the problems of water scarcity.

Page No 231:

Question 21:

 Describe one method of rainwater harvesting that is suitable for rural areas.

Answer:

A method of rainwater harvesting that is suitable for rural areas is construction of bunds or small check dams. The dams or bunds can trap rainwater or slow down its flow. This may slow down the run of water in lands where there are slopes, enabling stagnation and slow percolation. The rainwater that seeps into the ground ultimately reaches the aquifer and gets stored there.
By constructing dams, we can store water and can
contain excess water from causing trouble.

Page No 231:

Question 22:

Can we drink river water directly? Explain your answer.

Answer:

No, water obtained from river cannot be used directly for drinking as it contains impurities such as bacteria and other microorganisms. However, it can be made safe for drinking by boiling it for a few minutes.

Page No 231:

Question 23:

The numbered pictures on next page show the steps by which water is made suitable for human consumption. Rearrange the pictures in the correct order and write the numbers in the correct sequence.
figure

Answer:

The correct order of the pictures is:
a → f → b → g → e → h → c → d → i



Page No 232:

Question 24:

Answer each of the following questions briefly.
(a) Why is it not good to wash clothes in hard water?
(b) In what ways could the natural water cycle be affected if large forests are cut down?
(c) List at least four steps by which you can help to conserve water in your home.

Answer:

(a) It is not good to wash clothes in hard water because the soap does not form lather with hard water. In result, it forms a scum, which sometimes damages the fabric. Also, washing of clothes in hard water takes longer time to clean the cloth.

(b) Plants absorb carbon dioxide from the atmosphere to perform photosynthesis. If large forests are cut down, the level of carbon dioxide in the atmosphere will rise. As a result, carbon dioxide will trap more radiations, thereby adding to global warming. Global warming refers to an increase in earth’s temperature. An increase in earth’s temperature disturbs the natural water cycle. As a result, there will be a change in rainfall pattern, which in turn could lead to drought.

(c) Steps to conserve water at home:

  • Turn off water while brushing teeth.
  • Wash only full loads of dishes and clothes.
  • Don’t over water lawns and landscapes.
  • Take shorter showers.
  • Mop the floor instead of washing it.

Page No 232:

Question 25:

Sameer's mother found that they were getting muddy water in their taps. She collected the water and strained it through two layers of muslin cloth. Sameer told her that she needed to purify the water further, before they could drink it. He told her that there were two ways in which this could be done. Can you state what these two methods were?

Answer:

Two methods were as follows:

  1. Boiling of water for sometime may kill invisible microorganisms.
  2. Addition of potassium permanganate, which is a mild antiseptic  

Page No 232:

Question 26:

Find the mistakes in these equations and write them as correct balanced equations.
(a) Ca(HCO3) ⟶ Ca2CO3 + H2O + CO2
(b) Ca2(SO4)2 + Na2CO3 ⟶ CaCO3 + Na2SO4

Answer:

(a) Ca(HCO3)2 ⟶ CaCO3 + H2O + CO2

(b) CaSO4 + Na2CO3 ⟶ CaCO3 + Na2SO4



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