For a distillation to work the mixture needs to contain fractions that have different volatilities. The more volatile a component in a mixture is the higher the vapour pressure will be. Vapour pressure is determined by the kinetic energy of molecules. When the temperature reaches the boiling point, the average kinetic energy of the liquid particles is sufficient to overcome the force of attraction that hold the molecules in the liquid state.
Therefore, the concentration of the vapour above the distillation mixture will be higher with the more volatile component.
The boiling point of this particular fraction will determine where along the fractionating column this vapour will be condensed and converted to a liquid. When a compound in the gaseous state cools below its boiling point, it condenses into a liquid.
This seperation of mixtures is the basis for a simple distillation as the components have very different volatilities. Fractional distillation is used when the volatilities of the components are not very different and several successive distillations are needed.
Author: Lyndsey Booth (document modification date: 19th May 2003)