Evaporation And Transpiration

Water of Class 6

DISAPPEARING TRICK OF WATER: EVAPORATION AND TRANSPIRATION

When we spread wet clothes on a rope or wire (called clothes line), they dry up after some time. The water present in wet clothes seems to disappear. Actually, water present in wet clothes evaporates by receiving heat from the sun (or surrounding air) to form water vapour. This water vapour goes into air. Gradually, all the water present in wet clothes evaporates and

the clothes become dry. The changing of water into water vapour is called evaporation. The water spilled on the floor dries up and disappears due to evaporation. Similarly, water from wet roads and puddles near our homes and rooftops (formed during rains) also dries up and disappears by forming water vapour by the process of evaporation when the rain stops.
If we wipe a blackboard with wet cloth, it dries up after a while. The wet blackboard dries up after wiping it due to evaporation of water from its surface. When we iron (press) wet clothes, we can see steam (hot water vapour) arising from them. So, the steam arising from wet clothes, while they are being ironed, is also a process of evaporation of water.

Water needs heat to evaporate into water vapour. In the evaporation of water in nature, heat is pro-vided by the sun directly or by the surrounding air which has been heated by the sun. This will become more clear from the following activity.

If we want to dry our washed and wet school uniform quickly, we should spread it near a heater

(or an angithi). The heat produced by heater (or angithi) will evaporate the water present in wet clothes at a faster rate due to which the uniform will get dry quickly.

The evaporation of water can be made very fast by heating it on a burner. common salt dissolved in water can be separated by the process of evaporation. When water having salt dissolved in it is heated on a burner, water evaporates (turns into vapour) rapidly and goes into the air, but the salt is left behind. This is because though water forms vapour easily on heating, salt does not form vapour (or gas) at all. From this observation we conclude that the process of evaporation of salt water does not carry away the salt with it, the salt is left behind. We know that ocean water (or sea water) is saline which contains a lot of salts dissolved in it. So, when ocean water (or sea water) gets heated by the heat of the sun, then some of the ocean water evaporates to form water vapour which go into air. The salts present in the ocean water (or sea water) remain behind.

From the above discussion we conclude that when the sun shines, the heat of sun evaporates water from oceans, rivers, lakes, ponds and even soil continuously to form water vapour. This water vapour goes into the air (or atmosphere). The water vapour becomes a part of the air but it cannot be usually seen. In addition to evaporation, there is another process which puts a lot of water vapour into air. It is called transpiration. This is described below.

Loss of Water by Plants

Plants need water to grow. Plants take this water from the soil. Plants use a part of this water to make their food and retain some of it in their different parts. The remaining water is released by the plants into air as water vapour through the small pores in their layer (called stomata). The loss of water from plants as water vapour through the pores of their leaves is called transpiration. The process of transpiration puts a large amount of water vapour into the air.

Before we end this discussion, we can say that water vapour gets added to the air (or atmosphere)

by the processes of evaporation and transpiration. Evaporation of water takes place from oceans, rivers, lakes ponds and soil whereas transpiration occurs from the leaves of plants and trees. Thus, water vapour is always present in the air.

FORMATION OF CLOUDS : CONDENSATION

The mass of tiny droplets formed by the condensation water vapour which we see floating high

in the atmosphere is called a cloud. The formation of clouds involves the process of  condensation. The changing of water vapour into liquid water on cooling, is called condensation. We will now describe how clouds are formed. The formation of clouds will become clear from the following activity.

Condensation is the reverse of evaporation. We can see the process of condensation of water vapour to form liquid water in many instances of our daily life. For example, if we take out a bottle of ice-cold water from the refrigerator and keep it on a table, then a puddle of water is formed around it after some time. This is because when the water vapour present in air around the bottle comes in contact with the cold outer surface of bottle, it gets cooled and condenses to form liquid water which then forms a puddle of water around the bottle. We can see tiny drops of dew on the leaves of grass on the cold winter morning. The formation of dew is also due to the condensation of water vapour present m air. During cold winter morning many times we see a thick white cloud like thing near the ground which reduces our visibility this is called fog. Fog consists of a cloud of tiny water droplets suspended in the air near the ground. Fog is formed by the condensation of water vapour present in air near the ground during very cold winter mornings.

In order to clean their spectacles, people often breathe out on spectacle glasses to make them wet before wiping with a piece of cloth. The spectacle glasses become wet because the water vapour present in our breath condenses on the glasses to form tiny droplets of water. The process of condensation is involved in the formation of clouds in the sky and bringing back water to the surface of earth (in the form of rain, etc.). This happens as follows: Water vapour formed by the process of evaporation (from oceans, rivers, lakes, ponds and soil), as well as by transpiration (from plants and trees), goes into air.

The air containing water vapour is heated by the sun. Hot air, being lighter, rises high up in the atmosphere (or sky) Now, as we go higher and higher from the surface of earth, the temperature decreases, it becomes cooler and cooler. So, when the air containing water vapour rises to a high altitude in the atmosphere water vapour present in it gets cooled so much that it condenses to form tiny droplets of water. It is these tiny droplets of water, formed by the condensation of water vapour, which remain floating in air and appear to us as clouds. The tiny droplets of water in the clouds join together to form bigger droops of water. These drops of water fall down on the earth in the form of rain. During winter m S cold regions, the water drops in the sky freeze to form snow (ice). So, water also falls down to earth in the frozen state called snow. This is called snowfall. Snowfall occurs during winter in the extremely cold, hilly areas of the earth. When snow lying on high mountains melts, it forms water. Sometimes he frozen water from the clouds also falls on the earth in the form of small, round pieces of ice called hail.         

From the above discussion we conclude that water present on the earth forms water vapour by the processes of evaporation and transpiration, rises high up in the sky, gets cooled, condenses and comes back to earth mainly as rain and snowfall. As we will discuss after a while, rain and melting of snow replenishes (puts back) water in rivers, lakes, ponds, wells, soil as well as in oceans (from where it was taken originally).

BACK TO THE OCEANS

Only some of the water which falls on earth in the form of rain and snow is available to us in rivers lakes ponds and as groundwater. Most of the water that falls on earth as rain and snow sooner or later goes back to the oceans. Almost all the places on land are above the level of oceans (called sea-level). Since land is at a higher level, water can flow from land into the ocean (or sea). We will now desirable how the water which falls on land as rain or snow reaches the oceans.

The snow which falls on high mountains melts slowly to form water. This water then flows down the mountains in the form of streams and rivers. Some of the rain water which falls on land also goes into rivers. Most of the rivers cover long distances on land and ultimately fall into a sea or an ocean. This is how most of the water which falls on the earth as rain and snow goes back to the oceans. All the rain water, however, does not go back to oceans. The rain water also flows into lakes and ponds to fill them. It is also held by the soil. Some of the rain water which falls on earth seeps through the soil and goes under the surface of earth. Ultimately this water is stopped by some hard rocks and collects there. This water becomes available to us as ground water (or rather underground water). It is this ground water which we take out for our use by digging wells and tube-wells or by installing hand pumps.

From this discussion we conclude that water which was initially, taken away from rivers, lakes, pounds, oceans and soil etc., by evaporation and transpiration is finally put back into them. This happens over and over again leading to water cycle in nature.      

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