Why does fractional distillation work?

Fractional distillation relies on different fractions in the organic mixture having different boiling points. The difference in these boiling points could be caused by several things but the main determinant is the size of the molecule. Larger molecules require more energy to separate due to more intermolecular forces and subsequently have a higher boiling point. A distillation column is cooler at the top and warmer at the bottom. This means as the fractions rise through the column, larger molecules will condense lower down and be separated. Smaller molecules which will rise further up the column still in a gaseous state.