Define Vapour pressure | What is Vapour pressure

What is Vapour pressure?

Liquids occupy the shape of the vessel and exhibit a free surface since the liquid molecules have a high cohesive force. But in the case of gases and vapours, they fill the vessel since the binding force between molecules is extremely small in a gaseous state.

When the liquid is kept in a closed vessel, the molecules constantly leave the liquid surface and an equal number of molecules enter the liquid surface if no energy is added at a given temperature.

Under equilibrium conditions, the vapours above the liquid surface exert a pressure on the liquid surface called vapour pressure.