Evaporation: Concepts and Theories

Evaporation concepts and theories for school students.

Evaporation is the process by which a liquid changes into a gas or vapor at temperatures below its boiling point. When the liquid is heated formation of vapours takes place which appears in the form of bubbles.

Evaporation is a surface phenomenon.

Matter is composed of large number of tiny particles called molecules. However, not all the molecules of a liquid take part in evaporation. Only those molecules near the surface of the liquid which acquire sufficient Kinetic energy (K.E.) to escape as they overcome attractive forces of other molecules.

Then other molecules come to the surface of the liquid; these molecules acquire more K.E. by collision with the other molecules to reach the surface and escape from the surface. The molecules absorb heat from the surrounding and overcome the attractive forces of the other molecules inside the liquid to escape out in space.

This continues till all the liquid evaporates.

Average kinetic energy is the measure of the temperature of the body. As the liquid is heated to its boiling point, the temperature rises, leading to an increase in the average kinetic energy of the molecules. However, once the boiling point is reached, even though heat continues to be added, the temperature remains constant.

The added energy goes into overcoming intermolecular forces holding them together rather than increasing the kinetic energy further.

Thus, while there is an increase in average kinetic energy during the heating process leading up to boiling, at the boiling point itself, the average kinetic energy of the molecules of a liquid remains constant.

When boiling at a fixed temperature, the heat energy supplied to a liquid is used to cause the change in state from liquid to vapour. This energy, known as latent heat of vaporization, is absorbed by the liquid molecules to overcome the intermolecular forces holding them together. As the molecules gain this energy, they increase their potential energy, allowing them to break free and enter the gaseous state. Consequently, while heat is continuously supplied, the temperature (average kinetic energy) of the liquid remains constant during boiling.

Cooling is produced in the surrounding area when a liquid evaporates. Since evaporation requires heat, this heat is taken from the container or surrounding. Thus temperature of container or surrounding falls and cooling is produced.