Vapour Pressure

The intermolecular forces in a liquid prevent most molecules from escaping from the surface. However, due to molecular collisions, some molecules have sufficient kinetic energy to escape from the liquid.  This causes the evaporation of the molecules into the gaseous state.  Any liquid, therefore, has above its surface, a certain amount of material in the form of vapour. Vapour molecules move  in all directions. Some of the liquid get condensed.  When the rate of evaporation and condensation are equal, an equilibrium is established.  The pressure exerted by vapour above the liquid when equilibrium exists is vapour pressure.  The vapour pressure is temperature dependent.