Water is Chapter 16 taken from the JKBOSE textbook of Science for students of Class 7th. In the previous post, I provided you with Questions and Answers for Chapter 15 Light. This post is about Water Class 7 Question Answers. Before we proceed to Question Answers, you need to have basic information about the chapter. So here is a brief overview of the chapter:
Water Class 7 Question Answers
Overview of Chapter
- WATER AS A NATURAL RESOURCE.
- IMPORTANCE OF WATER FOR SUSTAINING LIFE.
- Water in living things.
- Water in the home.
- Water for irrigation.
- Water for Industry.
- Water for power.
- Water for transport and recreation.
- SOURCES OF WATER.
- Rivers and Lakes.
- Sea water.
- Groundwater.
- DIFFERENT STATES OF WATER.
- WATER AS A SOLVENT.
- WATER CYCLE IN NATURE.
- WATER POLLUTION.
- Causes of Water Pollution.
- Prevention of Water Pollution.
- POTABLE WATER.
- COMPOSITION OF WATER.
- PHYSICAL PROPERTIES OF WATER.
- RELATIVE VOLUME AND DENSITY OF WATER AND ICE.
- SALINITY OF SEAWATER.
- DESALINATION OF SEAWATER.
One cannot imagine life without water. Besides drinking, bathing and washing, water is used for many other purposes. It is used in the construction of houses. The different industries, factories and mills cannot run without water. They require plenty of water. The growth and development of plants can never be imagined without water.
WATER AS A NATURAL RESOURCE: Our water requirement depends entirely on nature. We have both surface and groundwater. It is a prime natural resource because it is an important medium for all life processes and acts as the universal solvent. Whenever the water cycle in nature is disrupted, the ecological balance in nature gets disturbed.
IMPORTANCE OF WATER FOR SUSTAINING LIFE: All living organisms on earth need water to survive. We are more dependent on water because we need it for our way of life. Water is important for sustaining life due to following reasons:
- Water in living things: The body of every organism consists mostly of water. Our body has about 65% water. The body of a mouse, an elephant and an ear of corn has about 70% of water. A potato and earthworm have about 80% water. Water is useful in the process of digestion, blood circulation and excretion etc. It also regulates body temperature.
- Water in the home: We need water for cleaning, washing, bathing, cooking and other activities at our home. On average, every person needs about 260 litres of water in a day.
- Water for irrigation: The majority of plants need great quantities of water. The crops need to be irrigated in dry areas to get enough food grains.
- Water for Industry: Water is used in an industry in many ways. It is used for the production process of many substances, for washing fruits and vegetables before canning and freezing them. It cools the stem used in the generation of electric power. It also cools hot gases produced in refining oil and hot steel produced from steel mills.
- Water for power: Thermal power stations use the steam of water to generate electricity while hydroelectric power projects use the kinetic energy of water to generate electricity.
- Water for transport and recreation: People still depend on water transportation to carry heavy and bulky products machinery such as machinery, coal, grain and oil. We build most of our recreation areas along lakes, rivers and seas. We enjoy water sports such as swimming, fishing and sailing.
SOURCES OF WATER: Taps, tube wells, water tanks, rivers, lakes, etc., are some sources of water. Rain is the main and ultimate source of water which give rise to other sources of water.
- Rivers and Lakes: Most cities that depend on rivers for water are located at the river banks. The amount of water in a river can vary from time to time. It depends on the amount of rainfall and the amount of snow that melts in the mountains.
- Sea water: Oceans are the largest reservoir of natural water because streams and rivers flow into them. But this water is not used for drinking, farming and manufacturing because it is too salty.
- Groundwater: The cities which are not near rivers or lakes use stored groundwater to meet their needs. Cities obtain underground water by drilling wells that reach below the water table and pumping up the water.
DIFFERENT STATES OF WATER: Water exists in three forms on earth. These are as follows:
- Solid (Snow or Ice): Water is present on earth in the form of snow and ice on snow-covered mountains, glaciers and ice caps on the poles of the earth.
- Liquid (Water): Water is present in this form in oceans, seas, lakes, rivers, streams and even underground water.
- Gas (Clouds or Water vapour): Water is present in water vapours form in the atmosphere. Water vapours on condensation form water droplets or snowflakes which precipitate on the earth’s surface in the form of rain and snow.
The existence of water in three physical states is mainly the function of temperature and pressure. The interconversion of water is brought about by the change in temperature and pressure.
WATER AS A SOLVENT: Water dissolves many types of substances in it. Rainwater after falling on the earth comes in contact with several minerals and salts and dissolves some of them. Hence it is called a universal solvent.
WATER CYCLE IN NATURE: Water circulation from the earth’s surface to the atmosphere and back to the earth is called the water cycle. Water falls on the earth in the form of rain, dew and snow etc. The water falls in ponds, streams, and oceans and some amount of water seeps into the earth. The heat of the sun evaporates water and changes it into water vapours. The water vapours rise up to a very cold zone in the atmosphere. These water vapours on cooling get condensed to form clouds. These clouds come back to earth in the form of rain and snow and this cycle is repeated again and again.
WATER POLLUTION: Water is termed as polluted when the quality or composition of water changes directly or indirectly as a result of man`s activities, so that it becomes less suitable for drinking purposes, domestic use, agricultural practice, fisheries or other purposes.
a) Causes of Water Pollution: There are several causes of water pollution. Some of them are as under:
- Domestic sewage.
- Industrial wastes.
- Agricultural wastes.
- Oil Pollution.
b) Prevention of Water Pollution: Here are some methods which can be used to prevent water pollution.
- Septic tank treatment should be used for houses.
- Biological treatment of sewage.
- Cleaning of sewage and factory waste before disposing of it in the water bodies.
- Minimise the use of fertilizers and pesticides.
- Reduce use of synthetic detergents.
- Do not throw dead bodies of animals and humans in water bodies.
- The water of rivers, streams, lakes and ponds should be purified.
- The excreta and other garbage should be treated in a biogas plant.
- Plant a large number of trees along the river banks.
POTABLE WATER: The water which is used for drinking purposes is known as potable water. It should be clean and fresh. It should not contain suspended or soluble impurities and harmful germs such as bacteria. Here are the steps which can be taken for the purification of water obtained from natural resources.
- Sedimentation.
- Filtration.
- Chlorination
- Aeration.
COMPOSITION OF WATER: Water is a compound. Its molecular formula is H2O. This formula tells us that a molecule of water is composed of two atoms of hydrogen and one atom of oxygen. The chemical composition of water is the same in all three states although it changes states but not the composition.
PHYSICAL PROPERTIES OF WATER: Here are the important physical properties of water:
- Nature: It is a colourless, tasteless and odourless liquid.
- Boiling point: Pure water boils at 100° C.
- Freezing Point: Water freezes at 0° C however under high pressure or it has impurities it freezes slightly less than 0° C
- Density: The density of water is 1g/cc at 4° C.
- Heat capacity: It is the ability to absorb heat without getting warmer. This property of water is used to cool engines and air coolers.
- Conduction: Pure water is a bad conductor of heat and electricity.
- Solubility: Water is termed a universal solvent because it dissolves almost any substance.
RELATIVE VOLUME AND DENSITY OF WATER AND ICE: Most substances contract when they freeze. In such substances, the density of the solid is higher than the density of the liquid. Again, water is an exception. It expands when it freezes; its density decreases. Ice floats on water because lower-density substances float in liquids of higher density.
SALINITY OF SEAWATER: Rainwater falling on earth flows in the form of rivulets, streams and rivers into the sea. As it 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. This result is the salination of seawater.
DESALINATION OF SEAWATER: The process of removing dissolved salts from saline water is called desalination. Desalination of seawater is done as follows:
- Sea water is collected in a huge tank.
- The tank is connected to another tank through a semi-permeable
- High pressure is applied from an external source on saline water.
- The pressure retains salts and allows only water to pass through a semipermeable membrane.
- Pure water collected in the tank is used to meet domestic needs, in industries and irrigation projects.
Chapter 16 Water Class 7 Question Answers
EXERCISES
(I). Answer the following questions in one word or a figure:
1. What is the apparatus used for the electrolysis of water called?
Ans. Hoffman’s Voltameter.
2. What is the volume ratio of hydrogen to oxygen in water?
Ans. The volume ratio for hydrogen to oxygen in water is 2:1.
3. At what temperature does water change into steam?
Ans. Water changes into steam at 100°C.
4. At what temperature does water change into ice?
Ans. Water changes into ice at 0° C.
5. At what temperature is the density of water maximum?
Ans. The density of water is maximum at 4° C.
6. What is the composition of water?
Ans. Water is composed of two atoms of hydrogen and one atom of oxygen.
7. Which is lighter: water or ice?
Ans. Ice is lighter than water.
8. Name the largest source of water on earth.
Ans. The largest source of water on earth is the ocean.
9. Name a liquid which is heavier than water.
Ans. Honey is heavier than water.
10. Name a chemical which is used to kill germs in the water.
Ans. Bleaching powder is used to kill germs in the water.
(II) Fill in the blanks.
- Water helps in the germination of seeds and fruits.
- About 97%of water is in the oceans.
- The property of fluidity and solubility makes water used in the process of digestion.
- The human body has about 70%of water by weight.
- Seeds cannot germinate without water.
- Removing dissolved salts from saline water is called desalination.
- The process of using electricity to split water molecules is called electrolysis.
- The density of liquid water is 1 g/cc.
- The density of water is maximum at 4°C.
- The freezing point of water is 100° C.
(III) Write ‘True’ or ‘False’ against each of the following statements:
- Ice has a lower density than water. (True)
- Rainwater is the purest form of natural water. (True)
- The solubility of gases in water increases with the increase in temperature. (False)
- During the summer, the amount of oxygen dissolved in water is increased and fish die. (False)
- Pure water tastes sweet. (False)
- The use of fertilizers pollutes water. (True)
- The ratio of oxygen to hydrogen in water is 2:1. (False)
- The states of water are interchangeable. (True)
(IV). Answer the following questions:
- State any three physical properties of water.
Ans. The three physical properties of water are:
a) Nature: It is a colourless, tasteless and odourless liquid.
b) Boiling point: Pure water boils at 100° C.
c) Freezing Point: Water freezes at 0° C however under high pressure or it has impurities it freezes slightly less than 0° C.
- How will you determine the freezing point of water?
Ans. The freezing point o water can be determined by the following method:
- Take some pieces of ice in a beaker.
- Stand a Celsius thermometer amongst them as shown in the figure.
- Measure the temperature of the ice.
- The ice melts and the thermometer show a constant temperature of approximately 0° C.
- The temperature remains the same till all the ice has melted.
This is the freezing point of water.
- How will you determine the boiling point of water?
Ans. The boiling point o water can be determined by the following method:
- Fill a beaker half with water.
- Fix a thermometer upright in a beaker using a stand in such a way that the bulb of the thermometer is immersed in water.
- Heat the beaker using a burner.
- After some time water in the beaker starts boiling.
- Note the temperature on the thermometer.
- The temperature shows 100 C at this time.
This temperature is called the boiling point of water.
- Briefly describe the water cycle in nature.
Ans. Water circulation from the earth’s surface to the atmosphere and back to the earth is called the water cycle. Water falls on the earth in the form of rain, dew and snow etc. The water falls in ponds, streams, and oceans and some amount of water seeps into the earth. The heat of the sun evaporates water and changes it into water vapours. The water vapours rise up to a very cold zone in the atmosphere. These water vapours on cooling get condensed to form clouds. These clouds come back to earth in the form of rain and snow and this cycle is repeated again and again.
- Mention five uses of water.
Ans. Five uses of water are as under:
- Water in living things: The body of every organism consists mostly of water. Water is useful in the process of digestion, blood circulation and excretion etc. It also regulates body temperature.
- Water in the home: We need water for cleaning, washing, bathing, cooking and other activities at our home. On average, every person needs about 260 litres of water in a day.
- Water for irrigation: The majority of plants need great quantities of water. The crops need to be irrigated in dry areas to get enough food grains.
- Water for Industry: Water is used in an industry in many ways. It is used for the production process of many substances, for washing fruits and vegetables before canning and freezing them.
- Water for power: Thermal power stations use the steam of water to generate electricity while hydroelectric power projects use the kinetic energy of water to generate electricity.
- How is water polluted?
Ans. Water is getting polluted due to the following reasons:
- Domestic sewage. Discharge of sewage water containing human urine, faeces, animal dung, cloth washing etc in water bodies causes water pollution.
- Industrial wastes. Toxic substances such as chloride, ammonia, different acids, and salts of metal like copper and zinc cause water pollution.
- Agricultural wastes. Minerals, organic wastes, and excessive pesticides that are carried to water bodies from agricultural fields also contribute to water pollution.
- Oil Pollution. Oil refineries near water bodies cause water pollution. The discharging of crude oil into the sea during transportation also causes water pollution.
- How can the pollution of water be prevented?
Ans. The pollution of water can be prevented in the following ways:
- Septic tank treatment should be used for houses.
- Biological treatment of sewage.
- Cleaning of sewage and factory waste before disposing of it in the water body.
- Minimise the use of fertilizers and pesticides.
- Reduce use of synthetic detergents.
- Do not throw dead bodies of animals and humans in water bodies.
- The water of rivers, streams, lakes and ponds should be purified.
- The excreta and other garbage should be treated in a biogas plant.
- Plant large number of trees along the river banks.
- What is electrolysis? Describe in brief the electrolysis of water.
Ans. The process of splitting the molecules using electricity is called electrolysis.
Electrolysis of water
Fill three fourth of a voltammeter with water. Add 2-3 drops of sulphuric acid to make it a good conductor of electricity.
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). We will see the bubbles rising to the top of the test tubes and gases collecting in them over the water.
We observe that 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 opening the cork. The gas burns with a popping sound and water are 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.
- Mention the names of some industries which pollute water.
Ans. Some industries which pollute water are:
- Oil refineries.
- Steel mill.
- Paint and Varnish industries.
- Leather and Wool industries.
- Synthetic detergents and fertilizers industries.
10. Give the names of some metals whose compounds are toxic.
Ans. Compounds of copper, zinc, lead, nickel, arsenic, mercury etc.
11. Why is saline water not suitable for drinking?
Ans. Saline water is not suitable for drinking because drinking salty water makes a person vomit.
12. What is potable water? Give its characteristics.
Ans. The water which is used for drinking purposes is known as potable water.
Characteristics of Potable Water:
It should be clean and fresh. It should not contain suspended or soluble impurities and harmful germs such as bacteria.
13. Name the elements that water is composed of.
Ans. Water is composed of two elements: a) Hydrogen and b) Oxygen.
14. Name two physical changes that take place in the water cycle.
Ans. The two changes that take in the water cycle are:
a) Evaporation.
b) Condensation.
15. How will you determine that a given sample of water is pure or polluted?
Ans. We can determine whether a given sample of water is pure or polluted by the following methods:
a) For Suspended Impurities: Suspend a piece of alum in the given sample of water. If some particles settle down then it shows the presence of suspended impurities in the sample.
b) For Dissolved Impurities: Take a few drops of water on a watch glass. Heat it on a burner flame to evaporate it to become dry. If some deposit is seen on the watch glass then it confirms the presence of soluble salt in the sample.
Determine the boiling point of a given sample of water. If it is 100°C then the given sample of water is pure. If it is more than 100°C Then it contains some dissolved salts in it.
c) Presence of germs and bacteria: Take a drop of a given sample of water on a watch glass. Observe it under the microscope for the presence of some germs. If they do not appear in the given sample then the sample of water is pure.
16. What is salinity? Describe the process of desalination.
Ans. The property of water containing soluble salts in it is termed salinity.
Desalination of seawater is done as follows:
a) Sea water is collected in a huge tank.
b) The tank is connected to another tank through a semi-permeable membrane.
c) High pressure is applied from an external source on saline water.
d) The pressure retains salts and allows only water to pass through a semipermeable membrane.
e) Pure water collected in the tank is used to meet the domestic need, in industries and irrigation projects.
V. Choose the correct answer:
- At which temperature is the density of water maximum?
(a) 0° C
(b) 4° C
(c) -4° C
(d) 100° C
Ans. (b) 4° C
- 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
- Which one of the following liquids has the greatest heat capacity?
(a) Water
(b) Glycerine
(c) Kerosene
(d) Oil
Ans. (a) Water.
- Which one of the following metals is not affected by water?
(a) Sodium
(b) Potassium
(c) Calcium
(d) Boron
Ans. (d) Boron
That’s all about Water Class 7 Question Answers. Hope it has helped you. Do share your views about this post in the comment section below.
Leave a Reply