Vapour Pressure

Molecules at the surface of a liquid constantly have enough energy to escape into the space above the liquid as vapour, a process called evaporation. If the liquid is placed in a sealed, closed container, evaporation continues alongside the reverse process, condensation (vapour molecules returning to the liquid); vapour pressure rises until the rate of evaporation exactly equals the rate of condensation, i.e. equilibrium is reached -- the pressure exerted by the vapour under this equilibrium condition is called the saturated vapour pressure, or simply vapour pressure, of the liquid (for water specifically, also called Aqueous Tension, see Table 10.5 in Section 10.5.6). Two main factors affect vapour pressure: the nature of the liquid -- liquids with weaker intermolecular forces have higher vapour pressure and are called volatile liquids (e.g. petrol evaporates faster than motor oil); and temperature -- heating a liquid raises its vapour pressure, since more molecules gain enough thermal energy to escape the su …