Liquid and vapour or gas (the vapour phase is created by supplying heat to the liquid) and mass is transferred from both the phase to one another, by vaporization from the liquid phase and by condensation from the vapour phase. The net effect is an increase in the composition of the more volatile component in the vapour (phase) and that of the less volatile component in liquid. The basic requirement for the separation of components by distillation is that the composition of the vapour be different from the composition of the liquid with which it is in equilibrium-the vapour is always richer in the more volatile component than the liquid from which it is formed. If the vapour composition is the same as the liquid compositions, distillation technique will not affect separation.