When a liquid is placed in an open vessel, it evaporates. The molecules in the liquid are moving with different kinetic energies. The molecules that possess above-average kinetic energies can overcome the intermolecular forces that hold them in the liquid. These energetic molecules escape from the liquid surface as vapour. The process by which molecules of a liquid go into the gaseous state (vapours) is called Vaporisation or Evaporation. The reverse process whereby gas molecules become liquid molecules is called Condensation. If the liquid is placed in a closed vessel the molecules with high kinetic energies escape into space above the liquid. As the number of molecules in the gas phase increases, some of them strike the liquid surface and are recaptured (condensation). A stage comes when the number of molecules escaping from the liquid is equal to the number of molecules returning to the liquid. In other words, the rate of evaporation exactly equalsthe rate of condensation. Thus a dynamic equilibrium is established between the liquid and thevapour at the given temperature. Liquid Vapour Now the concentration of the vapour in the space bove the liquid will remain unchanged with lapse of time. Hence the vapour will exert a definite pressure at the equilibrium. The vapour pressure of a liquid is defined as : the pressure exerted by the vapour in equilibrium with the liquid at a fixed temperature. The vapour pressures of various liquids differ considerably, depending upon the identity of the liquid with its particular intermolecular forces. Thus ethanol having weaker hydrogen bonding than
water, evaporates faster than water. Hence we expect that ethanol will have higher vapour pressure than water at a given temperature. As shown by the actual plot vapour pressure versus temperature, the vapour pressures of ethanol and water at 60ºC are about 350 torr and 150 torr respectively.