Question

What is the principle of evaporation?

Answer 1

Hint: Evaporation is the process by which the molecules of a liquid present on the surface that are exposed to the environment escape from the bulk medium into the atmosphere in the form of gases or vapour without heating the liquid up to its boiling point.

Complete answer:
Evaporation is a phenomenon involving the escape of the surface molecules of a liquid as vapours into the atmosphere at temperature below the boiling point of the liquid.
Evaporation is a surface phenomenon and is therefore affected by the kind of container in which the liquid is kept. When a larger part of the liquid is exposed to the atmosphere, the rate of evaporation enhances. The liquid particles on the top have higher thermal energy as compared to the particles that are trapped inside the bulk, thus the heat available in the surroundings is enough to allow them to break free from the liquid state by overcoming the forces of attraction and escape as gaseous particles. Little energy is needed by these particles to change their state.
The transformation of the surface liquid particles into vapours is spontaneous and the driving force for this conversion is the increase in overall entropy that occurs due to phase change.
Volatile liquids have a higher rate of evaporation as compared to non-volatile liquids as their particles require very low thermal energy to escape as vapours.
Thus the principle of evaporation involves the absorption of heat from the surroundings to facilitate a phase transformation and bring a cooling effect.

Note:
If evaporation is carried out in a closed container then the vapours do not get the opportunity to escape and condense on the surface of the lid covering the vessel. After some time an equilibrium is established and the rate of condensation becomes equal to the rate of evaporation.