Water – A Precious Resource | Science | Chapter 16 | Class 7th | Questions and Answers |

 

JANDKNCERT | Science | Class 7th |

Water – A Precious Resource | Science | Chapter 16 | Class 7th | Questions and Answers |

 

Chapter 16: Water – A Precious Resource

EXERCISES (Questions and Answers)

 

I. Answer the following questions in one word or a figure:

 

Q1. What is the apparatus used for the electrolysis of water called?

Ans. Hoffman’s Voltmeter

 

Q2. What is the volume ratio for hydrogen to oxygen in water?

Ans. 2:1

 

Q3. At what temperature does water change into steam?

Ans. 100⁰C

 

Q4. At what temperature does water change into ice?

Ans. 0⁰C

 

Q5. At what temperature is the density of water maximum?

Ans. 4⁰C

 

Q6. What is the composition of water?

Ans. H₂O

 

Q7. Which is lighter: water or ice?

Ans. Ice

 

Q8. Name the largest source of water on earth.

Ans. Oceans

 

Q9. Name a liquid, which is heavier than water.

Ans. Liquid wax

 

Q10. Name a chemical, which is used to kill germs in the water.

Ans. Chlorine

 

II) Fill in the blanks.

 

1. Water helps in the ________ of seeds and fruits.

2. About 97% of water is in the ________.

3. The property of ________ makes water, useful in the process of digestion.

4. The human body has about 70% of water by ________.

5. Seed cannot________ without water.

6. Removing dissolved salts from saline water is called________.

7. The process of using electricity to split water molecule is called ________.

8. The ________ of liquid water is 1 g/cc.

9. The density of water is maximum at ________.

10. The freezing point of water is ________.

 

Ans.

1. Water helps in the growth and development of seeds and fruits.

2. About 97% of water is in the oceans.

3. The property of fluidity and solubility makes water useful in the process of digestion.

4. The human body has about 70% of water by weight.

5. Seed cannot grow without water.

6. Removing dissolved salts from saline water is called desalination.

7. The process of using electricity to split water molecules is called electrolysis.

8. The density of liquid water is 1 g/cc.

9. The density of water is maximum at 4⁰C.

10. The freezing point of water is 0⁰C.

 Also Read: Waste, Water Story

III) Write 'True' or 'False' against each of the following statements:

 

1. Ice has a lower density than water. True

2. Rainwater is the purest form of natural water. True

3. The solubility of gases in water increases with the increase in temperature. False

4. During the summer, the amount of oxygen dissolved in water is increased and fish die. False

5. Pure water tastes sweet. False

6. The use of fertilizers pollutes water. True

7. The ratio of oxygen to hydrogen in water is 2:1. False

8. The states of water are interchangeable. True

 

IV. Answer the following questions:

 

Q1. State any three physical properties of water.

Ans. Physical properties of water:

1. Nature: Pure water is clear, colourless, tasteless, and odourless liquid.

2. Boiling point: Pure water boils at 100⁰C.

3. Density: The density of water is 1g/cc at 4⁰C.

 

Q 2. How will you determine the freezing point of water?

Ans. The freezing point of water can be determined by the following activity:

Take some pieces of ice in a beaker. Put a Celsius thermometer between these pieces, as shown in the figure. Measure the temperature of ice. The ice melts and the thermometer show a constant temperature of approximately 0⁰C. The temperature remains the same till all the ice has melted. This 0⁰C temperature is known as the point of water.

 

Q 3. How will you determine the boiling point of water?

Ans. The boiling point of water can be determined by the following activity:

Fill a beaker half with water. Use a stand to fix a thermometer upright in the beaker with the bulb of the thermometer emersed in the water. Heat the beaker. After some time, the water will start boiling. Note the temperature. At this, the thermometer shows a temperature of 100⁰C. this temperature is called the boiling point of water.

 

Fig: Boiling Point of Water

Q 4. Briefly describe the water cycle in nature.

Ans. During the daytime, sunlight falls on the water in oceans, rivers, lakes and ponds. The fields and other land areas also receive sunlight. As a result, water from all these places continuously changes into a vapour. Thus, evaporation takes place from all open surfaces of water. Water vapour enters the air through the processes of evaporation and transpiration. When the air moves up, it gets cooler and cooler. At sufficient heights, the air becomes so cool that the water vapour present in it condenses to form tiny drops of water called droplets. It is these tiny droplets that remain floating in the air and appear to us like clouds. Thus, water in the form of vapour goes into the air by evaporation and transpiration, forms clouds, and then comes back to the ground as rain, hail or snow. We now know that water brought back to the surface of the earth by rain, hail or snow, goes back to oceans. Thus, water from the ocean and surface of the earth goes into the air as vapour; returns as rain, hail or snow and finally goes back to the oceans. The circulation of water in this manner is known as the water cycle.

 

Fig: Water Cycle

Q 5. Mention five uses of water.

Ans. Five uses of water are as under:

i. It is used for cooking.

ii. It is used for drinking.

iii. It is used for bathing.

iv. It is used for washing.

v. It is used for agriculture.

 

Q 6. How is water polluted?

Ans. There are various causes of water pollution. Water is polluted by domestic sewage, industrial wastes, agricultural wastes, oil refineries, etc. Sewage water contains animal urine, faeces, cow dung, etc. Industrial wastes contain toxic substances like chloride, ammonia, hydrogen sulphide etc. Chemicals like fertilizers, insecticides, etc., from agricultural wastes, also pollute water.

 

Q 7. How can pollution of water be prevented?

Ans. Pollution of water can be prevented in the following ways:

1. Septic tank treatment should be used for individual houses of communities.

2. The sewage and factory wastes have to be cleaned before they flow into the water reservoirs.

3. Biological treatment for the sewage should be carried out.

4. The use of fertilizers and herbicides should be judicious and minimal.

5. The use of synthetic detergents should be minimized.

6. Dead bodies of humans and animals should not be thrown into the river.

7. The excreta and other garbage should be treated in a biogas plant to get fuel as well as manure.

8. The water of rivers, streams, lakes and ponds should be purified.

(This is done by the government. One such example is the Ganga Purification Project. The Government of India has undertaken the project to prevent the pollution of the Ganga river. Under this project, water treatment plants have been set up to treat the industrial and domestic wastewater entering the river.)

9. A large number of trees should be planted along the river banks.

 

Q 8. What is electrolysis? Describe, in brief, the electrolysis of water.

Ans. The method of using electricity to split the molecules of water into its constituent elements i.e., Hydrogen and Oxygen is called Electrolysis of Water.

Fill three fourth of a voltameter with water. Add 2-3 drops of sulphuric acid to make it a good conductor of electricity. Set up the apparatus as shown in the figure. Fill two test tubes with water and invert them over the two electrodes made of steel or carbon, not allowing any water in the test tube to spill. Connect the electrodes to the source of electricity (a battery). You will see the bubbles rising to the top of the test tubes and gases collecting in them over the water. Are the volumes of gases in both the tubes same? One test-tube seems to be collecting more gas than the other. After one of the test tubes is filled about half with gas, measure the height of the gas column in each test tube. The ratio of the collected gases seems to be 2:1. When one of the test tubes is filled with gas, remove it slowly and cork it. Now test this gas by bringing a burning splinter near its mouth and open the cork. The gas burns with a popping sound and, water is formed. This gas is Hydrogen. Similarly, test the gas from the other test tube when filled with gas by burning a glowing splint. The splint catches fire and burns brightly. This property is of Oxygen gas.

 

Fig: Electrolysis of Water

Q 9. Mention the names of some industries, which pollute water.

Ans. Fertiliser industry, Tannery industry, Plasticised industry and Chemical industry are the names of some industries that pollute water.

 

Q 10. Give the names of some metals whose compounds are toxic.

Ans. Arsenic, lead, mercury and fluorides are toxic metals whose compounds are present in industrial wastes.

 

Q 11. Why is saline water not suitable for drinking?

Ans. Saline water is not suitable for drinking because of the salt dissolved in it. Drinking salty water makes a person vomit, that is why saline water is not suitable for drinking.

 

Q 12. What is potable water? Give its characteristics.

Ans. The water, which is clean, fresh and free from all impurities is known as Portable Water. Portable water is safe for drinking.

 

Q 13. Name the elements that water is composed of.

Ans. Water is composed of two gaseous elements Hydrogen and Oxygen. It is a compound having the molecular formula, H₂O. The molecular formula of water shows that one molecule of water is composed of two atoms of hydrogen and one atom of oxygen.

H (gas) + O₂ (gas) ---à H₂O (water)

 

Q 14. Name two physical changes that take place in the water cycle.

Ans. Water-cycle is a physical process. Evaporation and condensation are the two physical changes that take place in the water cycle.

 

Q 15. How will you determine that a given sample of water is pure or polluted?

Ans. If the water is clear, colourless, tasteless, and odourless, then it is pure water. If the water is not clear then it is polluted water.

 

Q 16. What is salinity? Describe the process of desalination.

Ans. The salty nature of dissolved salts in water is known as salinity. Desalination is the process of removing dissolved salts from saline water. Following steps are taken for the desalination of water.

1. The seawater is collected in a huge tank.

2. The tank is connected with another tank through a semi-permeable membrane.

3. High pressure is applied from an external source on saline water.

4. The pressure retains salts and allows only water to pass through semipermeable membrane.

5. Pure water collected in the tank is used to meet the domestic need, in industries and irrigation projects.

Famous Lakes of Jammu and Kashmir State:   Famous lakes of Jammu Region: (i) Sanasar lake (ii) Mansar (iii) Saruinsar lake   Famous lakes of Kashmir Region: (i) Dal lake (ii) Wular lake   Famous lakes of Ladakh Region: (iii) Manasbal lake. (i) Pangong lake (ii) Tsomoriri lake

 

V. Choose the correct answer:

 1. At which temperature is the density of water maximum?

(a) 0 ⁰C

(b) 4 ⁰C

(c) -4 ⁰C

(d) 100 ⁰C

Ans. (b) 4 ⁰C

 

2. What is the volume ratio of oxygen to hydrogen in water?

(a) 1:2

(b) 1:3

(c) 2:1

(d) 3:1

Ans. (a) 1:2

 

3. Which one of the following liquids has the greatest heat capacity?

(a) Water

(b) Glycerine

(c) Kerosene

(d) Oil

Ans. (a) Water

 

4. Which one of the following metals is not affected by water?

(a) Sodium

(b) Potassium

(c) Calcium

(d) Boron

Ans. (d) Boron

 

Additional Questions from the Textbook

 

Worksheet 16.1 (Page No. 243)

 

1. By what per cent water is your body?

Ans. Our body is about 65 per cent of water.

2. By what per cent water is a potato?

Ans. A potato is about 80 per cent of water.

3. How much water do you require for different activities? Calculate the quantity of water used for different activities and put them in a tabular form.

Ans. A human body needs 2.4 litres of water a day. Each person may use an average of about 260 litres of water a day in the home.

 

Activities where water is used	Water is used in terms of litres
Bathing Drinking Washing clothes Cleaning utensils Cleaning house	115 – 150 litres 2.4 litres 100 litres 38 – 75 litres 200 litres

 

4. List any four uses of water in industries.

Ans. An industry uses water in the following ways.

i) It uses water in cleaning fruits and vegetable before canning and freezing them.

ii) It uses water as a raw material in soft drinks, canned foods and many other products.

iii) It uses water to air-condition and clean factories.

iv) It uses water for cooling.

5. What is the use of water in hydroelectric power stations?

Ans. Hydroelectric power stations use the energy of falling water from waterfalls and dams to turn the water turbines that drive a generator to produce electricity.

 

Worksheet 16.2 (Page No. 246)

 

1. Name the main source of water.

Ans. Rivers and lakes, seawater, and groundwater are the main sources of water.

2. Name the main source of water at your home.

Ans. Tape water

3. What is the source of underground water?

Ans. Rain is the main source of underground water.

4. Why do we not use seawater?

Ans. Sea water is salty. It cannot be used for drinking. Drinking salty water makes a person vomit.

5. From where does water come in rivers and streams?

Ans. Rivers and streams get water from rain and snow-covered mountains.

6. Define water table.

Ans. The upper limit of a saturated zone of ground where water gets collected is called as Water Table.

7. What is a spring? Is spring water drinkable?

Ans. The underground water that emerges out to the surface of the earth to form a flow of water is called a spring. Springwater is naturally filtered water and is safe for drinking, but still, open water should be boiled before it is used for drinking purposes.

 

Worksheet 16.3 (Page No. 248)

 

1. Name the three states of water.

Ans. Solid (ice), liquid (water), and gas (water vapour) are the three states of water.

2. Why do fish in shallow ponds die during summer?

Ans. During the summer, the water in the ponds gets hot due to summer heat, this results in the decrease of dissolved oxygen level in the water and hence the fishes die due to the lack of oxygen.

3. Name the factors that are responsible for the change of state of water.

Ans. The temperature and pressure are the factors responsible for the change of state of water.

4. Define temperature.

Ans. Temperature is the measure of the degree of hotness and coldness of an object.

5. Draw a diagram to explain the interchangeability of water.

Ans.

Fig: Interchangeability of Water

6. Explain the water cycle in nature.

Ans. See Q4 in the Exercise above.

7. Give examples to show that water is a universal solvent.

Ans. Water is called the "universal solvent" because it is capable of dissolving more substances than any other liquid. Wherever water goes, either through the air, the ground, or through our bodies, it takes along valuable chemicals, minerals, and nutrients. Gases like oxygen and carbon dioxide, which are insoluble in other liquids, are soluble in water.

8. Name two things, which cannot dissolve in water.

Ans. Stones, sand, oil, wax etc., are not dissolved in water.

9. At what temperature does the water boil?

Ans. Water boils at 100⁰C.

10. At what temperature does the water freeze?

Ans. Water freezes at 0⁰C.

 

Worksheet 16.4 (Page No. 251)

 

1. What is domestic sewage? What does it contain?

Ans. Domestic sewage is the contaminated or used water from houses and apartments, mostly coming from the kitchen, bathroom, and laundry sources. It contains animal excreta, chemicals like soap solutions, and household dirt.

2. Name the metals whose salts are contained in industrial wastes.

Ans. Copper, zinc, lead, nickel, mercury, arsenic are some metals whose salts are contained in industrial wastes.

3. How can you say that a given sample of water is polluted?

Ans. If the water is clear, colourless, tasteless, and odourless, then it is pure water. If the water is not clear then it is polluted water.

4. Why is the river water not fit for drinking purposes?

Ans. River water is unfit and unsafe for drinking because it contains domestic sewage and many impurities that make it unfit for drinking.

5. What are the properties of potable water?

Ans. It should be clean and fresh. It should not contain suspended or soluble impurities and harmful germs such as bacteria.

6. What is the importance of aeration for purifying water?

Ans. Aeration of water is important because it improves the taste and odour of water. In this process, water is usually sprayed through the air. The oxygen in the air takes away the bad taste and odour.

7. Why do we add alum to water tanks?

Alum is added to the water tank because it helps in quick settling of fine suspended materials that make the water partly clean.

 

Worksheet 16.5 (Page No. 256)

 

1. Which property of water makes the ice float on its surface?

Ans. The high density of water makes the ice float on its surface.

2. What is the formula of density?

Ans. The formula of the density of water is:

Density = Mass / Volume

3. Why is the seawater salty?

Ans. Sea water is salty due to the water cycle. During the water cycle, the rain flows over the land, many salts in the soil dissolve in it and are carried into the sea. Materials released by volcanoes and undersea springs also add salts to the sea.

4. How can seawater be used for irrigation?

Ans. Sea water can be used for irrigation after its desalination.

5. Name the element which is found in seawater but not in the water at high mountains.

Ans. Chlorine is the element found in seawater in abundance than the water at high mountains.