When a liquid is placed in a vessel and is covered with jar, from the liquid evaporation takes place and the vapour of the liquid or molecules of the liquid in gap form fill the available space. As the evaporation takes place over a period of time, the number of gaseous molecules goes up. As evaporation is taking place some molecules in the gaseous phase collide with the surface of the liquid and become liquid molecules. Thus both evaporation and condensation take place simultaneously. But initially there is more evaporation and less condensation. At the some stage, rate of evaporation equals rate of condensation and equilibrium is established between gas and liquid phases. The pressure exerted by the vapours at the equilibrium stage is called vapour pressure.
Definition
The pressure exerted by the vapours above the liquid surface (in a closed vessel) in equilibrium with the liquid at a given temperature is called vapour pressure.
Vapour pressure changes from liquid to liquid. It depends on intermolecular forces. if the forces in a liquid are weak, there is more gas formation and hence more vapour pressure.
A higher temperature there is more gas formation and hence for the same liquid vapour pressures increase with temperature.
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