How Does Evaporation Cause Cooling?
Evaporation is the method of turning of a substance into vapor or steam, well below its boiling level. Whatever the temperature at which evaporation occurs, when a liquid changes into vapor or gas, the latent heat of vaporisation must be supplied. This evaporation property is used to induce cooling.
Whenever a liquid evaporates, the latent heat of the vaporization must be supplied. The cooling caused by evaporation is based on the fact that when a liquid evaporates, the latent heat of vaporization is drawn or taken from anything that it touches. The substance from which this energy is drawn, gets cooled by losing heat.
Whenever we start to explain the cooling caused by evaporation it is necessary to keep the above points in mind. It is easier to explain this phenomenon with the help of some examples which are easily observed in our daily life.
1. If we put some spirit or petrol on the back of the hand and wave it around, the spirit quickly evaporates and our hands feel very cold. This is because spirit needs latent vaporization heat to move from liquid to vaporization state. The spirit takes this latent vaporizing energy out of our hand. The hand releases heat and becomes cold.
2. During hot summer days, water is usually kept in an earthen pot called picture or Matka to keep it cool.Water is usually stored in an earthen pot called pitcher or Matka for cooling during hot summer days. The earthen pot has in its walls a large number of extremely small pores, or hole. Some of the water continuously passes through these pores to the outside of the pot. This water evaporates continuously and takes from the earthen pot and the remaining water, the latent heat required for vaporisation. The remainder of the water thus loses heat and gets cold. This is also an example of the evaporation-induced cooling. It should be noted that due to the high value of latent heat of water vaporization all the water on earth does not get evaporated.
3. People often sprinkle water on the ground in front of their homes during the hot summer evening in many places, especially in villages.This water evaporates by removing from the ground and surrounding air, the large latent heat of vaporization. The place becomes cool and comfortable having lost heat. The water which vaporizes from trees’ leaves also cools the surrounding air in the same way.
4. Perspiration or sweating is the process by which our body holds a steady temperature.When our body temperature continues to increase too high on a hot day or after performing any vigorous activity, our sweat glands let out moisture or sweat on our bodies. When this sweat evaporates, our body takes the vaporizing latent heat from it. This keeps our body feeling cool.
5. In hot summer days we wear cotton clothing to keep us cool and relaxed. This can be explained by the cooling phenomenon which evaporation induces. In the hot summer days, we get lots of sweat on our body. Cotton is a strong water absorber, and it removes the moisture from our body and exposes it to evaporative weather. Our body cools down with this evaporation. The synthetic polyester clothes etc. don’t absorb much sweat so they don’t keep our body cool in the summer.A fan increases the rate of evaporation of sweat from our skin and makes us feel cool and comfortable.
6. On a hot and dry day a desert cooler cools better, because the desert cooler works on the evaporation principle. The higher temperature on a hot day decreases the rate of water evaporation, and the dryness often raises the rate of water evaporation. So, a desert cooler cools better on a hot and dry day because of the increased rate of water evaporation.
7. It’s a common observation that we can sip hot tea from a saucer faster than from a cup. This can be explained by the evaporation-induced cooling phenomena. Saucer has a considerable surface area. The evaporation of hot tea from the saucer is faster, due to the large surface area of the saucer. And this quicker evaporation cools the hot tea faster, making it easier for us to drink it.
The phenomenon of evaporation causes cooling is better understood by real life examples. The main working principle behind this observation is that whenever a substance converts into vapour or gas, it absorbs heat energy equal to the latent heat from the substance.