Fractional Distillation

Summary: A method of separating a mixture of miscible liquids with different boiling points using a fractionating column. Tags: igcse chemistry definition organic-chem practical-skills Created: 2026-07-14 Last Updated: 2026-07-14

Fractional distillation is a physical separation technique used to separate a mixture of two or more miscible liquids that have different boiling points, by means of a fractionating column that provides a temperature gradient. The mixture is heated in a flask; the vapour rises through the fractionating column, which is cooler at the top than at the bottom, causing vapours of higher-boiling components to condense and drip back down while the most volatile (lowest boiling point) component reaches the top and passes into the condenser to be collected as the first fraction. The fractionating column is typically packed with glass beads or has internal trays to provide a large surface area for repeated evaporation and condensation cycles, effectively performing many tiny distillations that greatly improve the separation compared to simple distillation. The most important IGCSE application is the industrial fractional distillation of crude oil in a tall tower, where petroleum is separated into useful fractions such as refinery gas, petrol, naphtha, kerosene, diesel, fuel oil, and bitumen, each collected at a different height corresponding to its boiling-point range. In the laboratory, fractional distillation is also used to separate ethanol from water after fermentation; exam questions often require students to label a fractionating column diagram, explain why it achieves better separation than simple distillation, and relate the boiling points of fractions to their molecular size and intermolecular forces.

Key term — IGCSE Chemistry (0620/0971) glossary.