Physics

Sound

Cooling Caused by Evaporation

We have already studied that whenever a liquid evaporates, it must be supplied the latent heat of vaporisation. The cooling caused by evaporation is based on the fact that when a liquid evaporates, it draws (or takes) the latent heat of vaporisation from 'anything' which it touches. By losing heat, this 'anything' gets cooled. Keeping these points in mind, we will now give some examples of the cooling caused by evaporation.
If we put a little of spirit (ether or petrol) at the back of our hand and wave it around, the spirit evaporates rapidly and our hand feels very cold. This is due to the fact that to change from the liquid to the vapour state, spirit requires latent heat of vaporisation. The spirit takes this heat of vaporisation from our hand. The hand loses heat and gets cooled. This is an example of cooling caused by evaporation.
During hot summer days, water is usually kept in an earthen pot (called pitcher or matka) to keep it cool. Let us see how it gets cooled. The earthen pot has a large number of extremely small pores (or holes) in its walls. Some of the water continuously keeps seeping through these pores to the outside of the pot. This water evaporates (changes into vapour) continuously and takes the latent heat required for vaporisation from the earthen pot and the remaining water. In this way, the remaining water loses heat and gets cooled. This is also an example of the cooling caused by evaporation. It should be noted that all the water on the earth does not get evaporated due to the high value of the latent heat of vaporisation of water. At many places, especially in villages, people often sprinkle water on the ground in front of their homes during the hot summer evenings. This water evaporates by taking the large latent heat of vaporisation from the ground and surrounding air. By losing heat, the place becomes cool and comfortable. Please note that water vaporising from the leaves of trees also cools the surrounding air.
Perspiration (or sweating) is our body's method of maintaining a constant temperature. On a hot day or after doing some physical exercise, when our body temperature tends to rise too much, our sweat glands give out moisture (or sweat) on our skin. When this sweat evaporates, it takes the latent heat of vaporisation from our body. This keeps our body cool.
We should wear cotton clothes in hot summer days to keep cool and comfortable. This can be explained as follows. We get a lot of sweat (pasina) on our body in hot summer days. Now, cotton is a good absorber of water, so it absorbs the sweat from our body and exposes it to the air (or atmosphere) for evaporation. The evaporation of this sweat cools our body. The synthetic clothes (made of polyester, etc.) do not absorb much of sweat, so they fail to keep our body cool in summer. A fan increases the rate of evaporation of sweat (or moisture) from our skin and makes us feel cool and comfortable.
The cooling in a desert room cooler is caused by the evaporation of water. A desert cooler cools better on a hot and dry day because the higher temperature on a hot day increases the rate of evaporation of water, and the dryness of air (low humidity of air) also increases the rate of evaporation of water. And due to increased rate of evaporation of water, a desert room cooler cools better on a hot and dry day.
It is a common observation that we are able to sip hot tea (or milk) faster from a saucer than from a cup. This can be explained as follows : Saucer has a large surface area. Due to the large surface area of saucer, the evaporation of hot tea (or milk) from the saucer is faster. And this faster evaporation cools the hot tea (or milk) more quickly making it convenient for us to drink it.

Notes

Sound - Notes

1. 3. Humidity of Air