Vapour Pressure of a Liquid

Liquids have an inherent tendency to evaporate: if a molecule at the liquid's surface has enough kinetic energy to overcome the intermolecular forces of attraction holding it in the liquid, it escapes into the vapour state. Evaporation happens specifically at the liquid's surface.

If evaporation takes place inside a closed container, the escaping vapour has nowhere to go -- it stays in contact with the liquid surface. These vapour molecules move about randomly, colliding with each other and with the container walls; because these collisions are inelastic, colliding vapour molecules lose energy, and some return to the liquid state. This reverse process is called condensation.

Evaporation and condensation both proceed continuously and simultaneously. In a closed system, a point is eventually reached where the rate of evaporation equals the rate of condensation -- a dynamic equilibrium between the liquid and its vapour. The pressure exerted by the vapour at this equilibrium is called the vapour pressure of the liquid, at that particular temperature. Vapour pressure depends on the liquid's chemical nature, on temperature (higher temperature gives higher vapour pressure, since more molecules have enough energy to escape), and, notably, does not depend on the surface area or the amount of liquid present (only on identity and temperature) -- once equilibrium is reached. …