Explain briefly the term "vapour pressure" . What are the factors on which vapour pressure of a liquid depends?

Vapour pressure: If a liquid is allowed to evaporate in a closed vessel, then molecules escaping from the liquid surface strike the walls of the container, rebound and may strike the surface of the liquid, where they are trapped. The return of the molecules from the vapour state to the liquid state is known as condensation. As evaporation proceeds, the number of molecules in the vapour state increases and, in turn, the rate of condensation increases. The rate of condensation soon becomes equal to the rate of evaporation i.e. the vapour in the closed container is in equilibrium with the liquid.

At equilibrium the concentration of molecules in the vapour phase and the pressure that it exerts remains unchanged. The pressure exerted by the vapour in equilibrium with liquid, at a given temperature, is called the vapour pressure of the liquid at that temperature.
Factors on which it depends:
(i) Nature of liquid. Liquids having weak intermolecular forces of attraction have a tendency to evaporate and therefore, have a high vapour pressure. On the other hand, liquids having strong intermolecular attractive forces have a small tendency to evaporate and therefore, have low vapour pressure.

(ii) Temperature. Vapour pressure of a liquid is directly proportional to its temperature. With the increase in temperature, the fraction of molecules having larger kinetic energy increases. Therefore, more molecules escape from the surface of the liquid. As a result, the concentration of molecules in the vapour phase will increase. Hence, the vapour pressure of the liquid increases.