The pressure exerted by the vapour of liquid on each other or other substance when they are in equilibrium with liquid state is called “VAPOUR PRESSURE”.
Vapour Pressure = force / Area
The unit of Vapour Pressure is N/m2 in MKS system.
Consider a liquid present in close container the molecules of liquid with higher energy escape from the surface of liquid, this process is called “EVAPORATION” While the vapors during motion collide with the liquid surface and return back in the liquid state, this process is called “CONDENSATION”. Initially the rate of evaporation is very high and the rate of condensation is very low.
As time passes the rate of evaporation continuously decrease while the rate of condensation continuously increases so that after a particular interval both rates are become equal to each other, this stage is called “EQUILIBRIUM”.