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14.3 WATER CYCLE
Disappearing Trick of Water How many times have you noticed that water spilled on a floor dries up after some time? The water seems to disappear. Similarly, water disappears from wet clothes as they dry up (Fig. 14.4). Water from wet roads, rooftops and a few other places also disappears after the rains. Where does this water go?rDo you remember Activity 6 in Chapter 5 in which water with salt dissolved in it was heated? What did we find? The water evaporated and the salt was left behind. This activity gives us an idea that, on heating, water change into its vapour. We also realise from this activity, that water vapour does not carry away the salt with it. Water vapours so formed become a part of the air and cannot usually be seen. We also found that heating is essential to convert water into its vapour. However, we have seen that water changes into its vapour alsofrom the fields, roads, rooftops and other land areas. We also discussed in Chapter 5 that to obtain salt, water from the sea is left in shallow pits to let the water evaporate. From where does this water get the heat it needs to evaporate?
Let us find out. Suppose for some reason your family gets only one bucket of water everyday for a week. Imagine what would happen? Would you be able to cook, clean utensils, wash clothes or bathe? What are the other activities you would not be able to do? What would happen if we do not have easy access to water for a long period of time?
Apart from drinking, there are so many activities for which we use water (Fig. 14.1). Do you have an idea about the quantity of water we use in a single day?each activity by you and other family members. You may use a mug, a glass, a bucket or any other container t measure the amount of water used.
14.1 HOW MUCH WATER DO WE USE?
List all the activities for which you use water in a day. Some activities are listed in Table 14.1. Make a similar table in your notebook. Throughout the day, measure the amount of water used for Table 14.1 Estimation of the amount of water used by a family in a day.You now have a rough idea as to how much water your family uses in a day. Using this information, calculate the amount of water needed by your family in a year. Now, divide this amount by the number of members of your family. This will give an idea of the amount of water needed by one member of your family in a year. Find the number of people that live in your village or town.
water from a river, spring, pond, well or a hand pump”. Some others might say, “We get water from taps”. Have you ever wondered where water in the taps comes from? Water that we get from taps is also drawn from a lake or a river or a well (Fig. 14.2). It is then supplied through a network of pipes.You have listed a number of activities for which you use water. Do you think, our water requirement is limited to activities like these? We use wheat, rice, pulses, vegetables and many other food items everyday. We know that some o the fibres that we use for making fabriccome from plants. Is water not needed to grow these? Can you think of some more uses of water? Water is used in industries for producing almost all thethings that we use. So, we need water not only for our daily activities but also for producing many things.Each of us may be getting water into our homes in different ways. But, finally, all of us get water from the same sources such as ponds, lakes, rivers and wells. We have discussed some of the sources of water. Where does the water come from, to fill these ponds, lakes, rivers and wells?
1. Fill up the blanks in the following:
(a) The process of changing of water into its vapour is called _________________.
(b) The process of changing water vapour into water is called _________________.
(c) No rainfall for a year or more may lead to _________________ in that region.
(d) Excessive rains may cause _________________.
2. State for each of the following whether it is due to evaporation or condensation:
(a) Water drops appear on the outer surface of a glass containing cold water.
(b) Steam rising from wet clothes while they are ironed.
(c) Fog appearing on a cold winter morning.
(d) Blackboard dries up after wiping it.
(e) Steam rising from a hot girdle when water is sprinkled on it.
3. Which of the following statements are “true” ?
(a) Water vapour is present in air only during the monsoon. ( )
(b) Water evaporates into air from oceans, rivers and lakes but not from the soil.( )
(c) The process of water changing into its vapour, is called evaporation.( )
(d) The evaporation of water takes place only in sunlight.( )
(e) Water vapour condenses to form tiny droplets of water in the upper layers of air where it is cooler.( )
4. Suppose you want to dry your school uniform quickly. Would spreading it near an anghiti or heater help? If yes, how?
5. Take out a cooled bottle of water from refrigerator and keep it on a table. After some time you notice a puddle of water around it. Why?
6. To clean their spectacles, people often breathe out on glasses to make them wet. Explain why the glasses become wet.
7. How are clouds formed?
8. When does a drought occur?
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