Solved examples for class 7 science : Chapter 16

    Water: A Precious Resource

    I. Multiple Choice Questions (MCQs).
    Q.1. Which of the following day is celebrated as World Water Day?
    a) 22 March
    b) 20 March
    c) 22 February
    d) None of these
    Answer: (a) 22 March
    Q.2. What percentage of earth′s surface is covered by water?
    a) 66%
    b) 71%
    c) 75%
    d) None of these

    Answer: (b) 71%
    Q.3. The largest reservoir of potable water is
    a) rivers
    b) ocean
    c) glaciers
    d) none of these
    Answer: (c) glaciers
    Q.4. Which of the following years was observed as the international year of freshwater to make people aware of global shortage of usable water?
    a) Year 2000
    b) Year 2002
    c) Year 2003
    d) Year 2005
    Answer: (c) Year 2003
    Q.5. The solid state of water is found in
    a) glaciers
    b) icebergs
    c) hails
    d) all of these
    Answer: (d) all of these
    II. Fill in the blanks by selecting appropriate words from the given list :
    List : 50, weather, wastage, transpiration, scarcity
    Q.1. There is ______________ of usable water in large parts of the world.
    Q.2. The amount of water recommended by United Nations for drinking, cooking, washing and for maintaining proper hygiene is minimum of ______________ litres per person per day.
    Q.3. Plants release water vapour during ____________ from the surface of leaves.
    Q.4. Water cycle helps in regulating ______________ on the earth.
    Q.5. We need to think of various ways to avoid the ______________ of water.
    1- scarcity ,
    2- 50
    3- transpiration,
    4- wastage,
    5- weather

    III. Match the items of Column A with those of Column B.

    Answer: 1-(e) 2-(a) 3-(d) 4-(b) 5-(c)
    IV. Write ‘T’ for the true and ‘F’ for the false statements.
    Q.1. Earth′s atmosphere contains a large amount of water vapour.
    Q.2. The water vapours rise up to a certain height where they condense to form clouds.
    Q.3. Deforestation increases seepage area of water.
    Q.4. Water saving can be done by civic authorities only.

    Q.5. Water evaporates all the time from the surface of water bodies due to heat of the sun.
    Answer. 1- T, 2- T, 3-F, 4-F, 5-T
    V. Very Short Answer Questions.
    Q.1. What is responsible for continuous circulation of water in nature?
    Answer: Water cycle is responsible for continuous circulation of water in nature.

    Q.2. Write the purest form of natural water.
    Answer: Rainwater
    Q.3. How does water drawn from the ground get replenished?
    Answer: Water drawn from the ground gets replenished by the seepage of rainwater.

    Q.4. Name an improved method of irrigation.
    Answer: Drip irrigation.

    Q.5. Name two diseases caused by polluted water.
    Answer: Jaundice and hepatitis

    VI. Short Answer Questions.

    Q.1. What do you mean by water table?
    Answer: The upper Level of groundwater is called water table. The water table varies from place to place.

    Q.2. Why is rainwater harvesting needed? Write its different ways.
    Answer: Rainwater harvesting system helps us to use rainwater directly. Rainwater can be used to recharge the groundwater. This water harvesting can store rainwater during the monsoon season and serves as a source of drinking and irrigation during the rest of the year.
    ▪ Surface runoff harvesting
    ▪ Roof top rainwater harvesting

    Q.3. Define different sources of water with proper examples. Ans: Water in nature occurs in two forms:

    (a) Surface water: It is present on the surface of earth. The sources of surface water are lakes, rivers, streams and oceans.

    (b) Underground or groundwater: Rainwater that penetrates through the soil get collected over the rocky bed present underground to form reservoir of water called groundwater or underground water. Spring water and well water are example of ground water.

    Q.4. What is the importance of water cycle? Draw the diagram of water cycle.
    Answer: Importance of Water cycle:
    • Water cycle helps in regulating weather on the earth.
    • Water cycle makes water available in various forms on the earth.

    Q.5. What are the main causes of depletion of water table? Describe briefly. Answer: Following are the main causes of depletion of water table:

    (a) Increasing population: It creates demand for construction of houses, commercial buildings, mall, offices and other infrastructures. As a result, it decreases the open areas like parks and playgrounds for the seepage of rainwater into the ground. This leads to consume more groundwater and allow lesser water to seep into the ground. This causes the depletion of water table.
    (b) The increasing industries are also responsible for depletion for water table, because industries draw maximum water from the ground.
    (c) Agricultural activities: Farmers have to use groundwater for their cultivation because rainwater is not sufficient to irrigate their crops. As a result, depletion of water table takes place.
    (d) Deforestation: The complete or partial removal of trees and vegetation from an area of land is called deforestation. It decreases seepage area of water which is also responsible for depletion of water table.

    Q.6. (a) What do you understand by water management?
    (b) What is the effect of water scarcity on plants?
    Answer (a) The wise use and management of water in a manner that provides the maximum benefit to all life a period of time, is called conservation of water. So, we need to think of various ways to avoid the wastage of water.

    (i) Households can save water by promptly fixing Leaks or dropping taps
    (ii) Recycling of water is very important, water which is used in cleaning utensils can be used to water plants in the garden.

    (iii) By improving the methods of irrigation: Drip irrigation technique of watering plants by making use of narrow tubings which deliver water directly at the base of the plant.
    (iv) Rainwater harvesting system: This system helps us to use rainwater directly. Rainwater can be used to recharge the groundwater.

    b) Scarcity of water is defined as a situation in which water is insufficient to satisfy normal requirements. In the absence of water, wilting of plants will take place and in severe conditions plant may die. Plants can absorb nutrients from the soil only in the preservence of water. Without water survival of plants is not possible and in absence of plants, life cannot exist on the earth.

    VII. Long Answer Questions.
    Q.1. (a) Describe some methods of purification of water at home.
    (b) Describe some methods of purification of water for city supply.

    Answer (a) Following are the some methods of purification of water at home.

    (i) By filtration –Suspended impurities from the water can be removed by filtering water through a fine muslin cloth.
    (ii) By boiling –The filtered water can be made germ-free by boiling for 10-15 minutes and cooling it before use.
    (iii) By treating with some chemicals – The filtered water can also be made germ-free by treating the water with bleaching powder or chlorine tablets.
    (iv) By exposing water to ultraviolet radiation –The filtered water can be made germfree by exposing it to ultraviolet radiation.

    (b) Following are the some methods of purification of water for city supply.

    (i) Removal of suspended impurities – The pumped water is mixed with a small quantity of alum and allowed to stand for some time. The suspended particles of clay settle down at the bottom of tank and upper layer is sent for filtration.
    (ii) Filtration – After sedimentation water is passed through thick layers of sand and gravel so that fine suspended impurities get removed.
    (iii) Aeration – Air under pressure is then blown into the filtered water. This process is called aeration. Harmful microorganisms present in the water are killed through this process.
    (iv) Chlorination – The filtered and aerated water is chlorinated by adding chlorine to it. The process of chlorination makes the water germfree. This purified water is supplied to the users through a network of water pipes.

    Q.2. What do you understand by water pollution? What are its causes? Describe how to control water pollution briefly.

    Answer: Contamination of water by harmful substances, germs, etc., is called water pollution. The substance causing pollution is called a pollutant. Sewage, garbage, animal dung, pesticides and toxic chemicals are various pollutants.

    Causes of Water Pollution
    (i) Disposal of untreated sewage into rivers, Lakes, etc.: The untreated sewage contains harmful microorganisms which cause various diseases such as jaundice, dysentery, diarrhoea, etc.
    (ii) Disposal of untreated industrial waste: Many industries and factories using chemicals, dispose off their wastes into rivers, canals, lakes, etc., these chemicals are highly toxic and poisonous to the organisms.
    (iii) Excessive use of fertilisers and pesticides: Fertilisers and pesticides are nonbiodegradable substances. When the farmers use these chemicals in their fields, these are washed off by rain and carried to water bodies because these chemicals are highly soluble in water. Thus, excessive use of fertilisers and pesticides may be harmful for aquatic plants and animals.
    Control of water Pollution
    i Domestic sewage and industrial waste must be pre-treated before it is discharged into a lake or a river.
    ii Human and animal’s excreta should be prevented from mixing with water resources.
    iii Avoid excessive use of pesticides and fertilizers.
    iv We should not wash clothes, clean utensils and take bath near the sources of drinking water.
    v A covering and a wall should be made around the well.

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