Water S - NCERT

14WaterSuppose for some reason yourfamily gets only one bucket ofwater everyday for a week.Imagine what would happen? Would yoube able to cook, clean utensils, washclothes or bathe? What are the otheractivities you would not be able to do?What would happen if we do not haveeasy access to water for a long periodof time?Apart from drinking, there areso many activities for which we usewater (Fig. 14.1). Do you have an ideaabout the quantity of water we use in asingle day?each activity by you and other familymembers. You may use a mug, a glass,a bucket or any other container tomeasure the amount of water used.Table 14.1 Estimation of theamount of water used by yourfamily in a dayActivityAmount ofwater usedDrinkingBrushingBathingWashing utensilsWashing clothesToiletsCleaning floorAny otherTotal water used ina day by a familyFig. 14.1 Uses of water14.1 HOW MUCH WATER DOWEUSE?Activity 1List all the activities for which you usewater in a day. Some activities are listedin Table 14.1. Make a similar table inyour notebook. Throughout the day,measure the amount of water used forYou now have a rough idea as to howmuch water your family uses in a day.Can you estimate the amount of waterused by you for personal cleanliness ina day. Using this information, calculatethe amount of water needed by yourfamily in a year. Now, divide this amountby the number of members of yourfamily. This will give an idea of theamount of water needed by one memberof your family in a year. Find the numberof people that live in your village or town.SCIENCE1362021–22

You may now get an idea of the amountof water needed by your village or townin a year.Boojho wonders whether peopleliving in different regions of ourcountry get the same amount ofwater. Are there regions wherepeople do not get adequate amountof water? How do they manage?You have listed a number of activitiesfor which you use water. Do you think,our water requirement is limited toactivities like these? We use wheat, rice,pulses, vegetables and many other fooditems everyday. We know that some ofthe fibres that we use for making fabriccome from plants. Is water not neededto grow these? Can you think of somemore uses of water? Water is used inindustries for producing almost all thethings that we use. So, we need waternot only for our daily activities but alsofor producing many things.water from a river, spring, pond, well ora hand pump”. Some others might say,“We get water from taps”. Have you everwondered where water in the taps comesfrom? Water that we get from taps is alsodrawn from a lake or a river or a well(Fig. 14.2). It is then supplied througha network of pipes.Fig. 14.2 Water in taps comes from rivers,lakes, borewell or wellsEach of us may be getting water intoour homes in different ways. But, finally,all of us get water from the same sourcessuch as ponds, lakes, rivers and wells.We have discussed some of thesources of water. Where does the watercome from, to fill these ponds, lakes,rivers and wells?Paheli wants to tell you thatabout two glasses of waterare required to produceeach page of a book.14.2 WHERE DO WE GET WATERFROM?Where do you get the water that youuse? Some of you may say, “We drawWATERBoojho wants you to imagine aday in your life when water supplythrough taps is not available. So,you have to fetch it yourself froma far away place. Would youuse the same amount of wateras on any other day?1372021–22

That is where the water cyclecomes in!14.3 WATER CYCLEDisappearing Trick of WaterFig. 14.3 Oceans cover a major part of the earthDo you know that about two thirdsof the Earth is covered with water? Mostof this water is in oceans and seas(Fig. 14.3).The water in the oceans and seas hasmany salts dissolved in it — the wateris saline. So, it is not fit for drinkingand other domestic, agricultural andindustrial needs. You might have heardthe famous lines of the poem “Rime ofthe Ancient Mariner” written by S.T.Coleridge in 1798:How many times have you noticed thatwater spilled on a floor dries up aftersome time? The water seems to disappear.Similarly, water disappears from wetclothes as they dry up (Fig. 14.4). Waterfrom wet roads, rooftops and a few otherplaces also disappears after the rains.Where does this water go?“Water water every whereNor any drop to drink”Fig. 14.4 Clothes drying on a clothes-lineHere the poet has described theplight of sailors on a ship lost in theocean.Yet, oceans play an important rolein supplying the water that we use. Doyou find this surprising? After all, thewater that we use is not salty. Many ofus live in places far away from theoceans. Does the water supply in theseplaces also depend on the oceans? Howdoes the ocean water reach ponds, lakes,rivers and wells, which supply us water?How come the water from these sourcesis not saline anymore?Do you remember Activity 6 inChapter 5 in which water with saltdissolved in it was heated? What did wefind? The water evaporated and the saltwas left behind. This activity gives usan idea that, on heating, water changesinto its vapour. We also realise from thisactivity, that water vapour does not carryaway the salt with it. Water vapours soformed become a part of the air andcannot usually be seen. We also foundthat heating is essential to convert waterinto its vapour. However, we have seenthat water changes into its vapour alsoSCIENCE1382021–22

from the fields, roads, rooftops andother land areas. We also discussed inChapter 5 that to obtain salt, water fromthe sea is left in shallow pits to let thewater evaporate. From where does thiswater get the heat it needs to evaporate?Let us find out.Activity 2Take two similar plates. Place one of theplates in sunlight and keep the otherunder shade. Now, pour equal amountof water in each of the plates (Fig. 14.5).You can use a cap of a bottle to measurewater. Make sure that water does notspill over. Observe the two plates afterevery 15 minutes. Does the water seemto disappear? From which plate does itdisappear first? What is the source ofheat for this evaporation?During the daytime, sunlight falls onthe water in oceans, rivers, lakes andponds. The fields and other land areasalso receive sunlight. As a result, waterfrom all these places continuouslychanges into vapour. However, the saltsdissolved in the water are left behind.In Activity 2, did you find that wateralso disappeared from the plate kept inthe shade, though it could have takenmore time? Does the heat from thesunlight reach here? Yes, during thedaytime all the air surrounding us getsheated. This warm air provides heat forevaporation of water in the shade. Thus,evaporation takes place from all opensurfaces of water. As a result, watervapour gets continuously added to air.However, evaporation of water is a slowprocess. That is why we rarely notice itsloss from a bucket full of water. Insunlight, evaporation takes place faster.On heating water on a burner, itsevaporation takes place even faster. Isthere any other process through whichwater vapour gets transferred into air?Loss of Water by PlantsYou have learnt in Chapter 7 that plantsneed water to grow. Plants use a part ofthis water to prepare their food andBoojho has been reading abouttranspiration. He asked himself how much water is lost throughtranspiration by wheat plantsthat give us one kilogram ofwheat? He found out that this isnearly 500 litres, that is, roughly25 large sized buckets full ofwater. Can you now imagine theamount of water lost by plantsof all the forests, crops andgrasslands together?Fig.14.5 Evaporation of water in sunlight and inshadeWATER1392021–22

retain some of it in their different parts.Remaining part of this water is releasedby the plants into air, as water vapourthrough the process of transpiration. Doyou remember observing transpirationof water by plants in Activity 4 inChapter 7?Water vapour enters the air throughthe processes of evaporation andtranspiration. Is it lost for ever? No, weget it back again, as we will see.Paheli has noticed dew on leaves ofgrass on winter mornings. Did younotice something similar on leavesor metal surfaces like iron grills andgates on a cold morning? Is thisalso due to condensation? Do yousee this happening on hotsummer mornings?How are clouds formed?Activity 3Take a glass half filled with water. Wipethe glass from the outside with a cleanpiece of cloth. Add some ice into thewater. Wait for one or two minutes.Observe the changes that take place onthe outer surface of the glass (Fig.14.6).From where do water drops appearon the outer side of the glass? The coldsurface of the glass containing iced water,cools the air around it, and the watervapour of the air condenses on thesurface of the glass. We noticed thisprocess of condensation in Activity 7 inChapter 5.The process of condensation plays animportant role in bringing water backto the surface of earth. How does ithappen? As we go higher from thesurface of the earth, it gets cooler. Whenthe air moves up, it gets cooler andcooler. At sufficient heights, the airbecomes so cool that the water vapourpresent in it condenses to form tinydrops of water called droplets. It is thesetiny droplets that remain floating in airand appear to us as clouds (Fig. 14.7).It so happens that many droplets ofwater come together to form larger sizedIceWaterdropletsFig. 14.6 Drops of water appear on outersurface of the glass containing water with iceFig. 14.7 CloudsSCIENCE1402021–22

Boojho has noticed fog near theground in winter mornings. Hewonders if this is also condensationof water vapour near theground. What do youthink?drops of water. Some drops of waterbecome so heavy that they begin to fall.These falling water-drops are, what wecall rain. In special conditions, it mayalso fall as hail or snow.Thus, water in the form of vapourgoes into air by evaporation andtranspiration, forms clouds, and thencomes back to the ground as rain, hailor snow.14.4 BACKTO THEOCEANSWhat happens to the water that rain andsnow bring to different regions of earth?Almost all land surfaces are above thelevel of oceans. Most of the water thatfalls on the land as rain and snowsooner or later goes back to the oceans.This happens in many ways.Snow in the mountains melts intowater. This water flows down themountains in the form of streams andrivers (Fig. 14.8). Some of the water thatfalls on land as rain, also flows in theform of rivers and streams. Most of therivers cover long distances on land andultimately fall into a sea or an ocean.However, water of some rivers flowsinto lakes.Fig. 14.8 Rainwater flows down in the form ofstreams and riversThe rainwater also fills up the lakesand ponds. A part of the rainwater getsabsorbed by the ground and seems todisappear in the soil. Some of this wateris brought back to the air by the processof evaporation and transpiration. Therest seeps into the ground. Most of thiswater becomes available to us as groundwater. Open wells are fed by groundwater. Ground water is the source formany lakes as well. It is also this groundwater which is drawn by a handpumpor a tubewell. The more handpumps ortubewells that are used in an area, thedeeper we need to dig to find this groundwater. The loss in the level of groundwater due to over use, is worrisome.Paheli wants to share a concern withyou. In those areas where the land haslittle or no vegetation, the rainwater flowsaway quickly. Flowing rainwater alsotakes the top layer of the soil away withit. There are few areas where most of theland is covered with concrete. Thisreduces the seepage of rainwater into theground which ultimately affects theavailability of ground water.WATER1412021–22