The process of a liquid changing into vapour (or gas) even below its boiling point is called Evaporation. Whatever be the temperature at which evaporation takes place, the latent heat of vaporization must be supplied whenever a liquid changes into vapour (or gas).
The process of evaporation can be explained as follows. Some particles in a liquid always have more kinetic energy than the others. So, even when a liquid is well below its boiling point, some of its particles have enough energy to break the forces of attraction between the particles and escape from he surface of the liquid in the form of vapour (or gas). Thus, the fast moving particles (or, molecules) of a liquid are constantly escaping from the liquid to form vapour (or gas), which is known as evaporation.
Factors Affecting Evaporation
The evaporation of a liquid depends mainly on the following factors:
1.Temperature: The rate of evaporation increases on increasing the temperature of the liquid.
2.Surface area of the liquide rate of evaporation: The rate of evaporation icreasesing on increasing the surface area of the liquid.
3.Humidity of air: Humidity of air tells us the degree of 'wetness' of air.
When the humidity of air is low, then the rate of evaporation is high, and water evaporates more readily.
4.Wind speed: The rate of evaporation of a liquid increases with increasing wind speed.
5.Nature of liquid: Lower the boiling point of a liquid, higher is its rate of evaporation. For example, the boiling point of water is 100°C while that of alcohol is about 78°C. Hence, alcohol evaporates faster than water.
Cooling Caused by Evaporation
The cooling caused by evaporation is based on the fact that when a liquid evaporates, it draws (or takes) the latent heat of vaporization from 'anything' which it touches.
If we put a little of spirit (acetone) at the back of our hand, 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 vaporization. The spirit takes this heat of vaporization from hand. The hand loses heat and gets cooled. This is an example cooling caused by evaporation.