Crude oil is separated by fractional distillation. Explain how fractional distillation separates crude oil into fractions, and state two ways in which a short-chain hydrocarbon fraction differs in its properties from a long-chain hydrocarbon fraction.

Crude oil is heated in a fractionating column until the hydrocarbons evaporate. The vapour rises and is cooled at different heights of the column, so that hydrocarbons with lower boiling points condense first and are collected as the top fractions, while those with higher boiling points condense later and are collected lower down. Each fraction therefore contains hydrocarbons with a similar number of carbon atoms.

Short‑chain hydrocarbons have a lower boiling point than long‑chain hydrocarbons and are therefore more flammable. They are also less viscous (more runny) than long‑chain hydrocarbons.