Describe how fractional distillation is used to separate crude oil into fractions, and explain why fractions collected near the top of the fractionating column are more useful as fuels than those collected near the bottom.

Crude oil is heated in a distillation column until the hydrocarbons evaporate. The vapour rises through the column and, because the hydrocarbons have different boiling points, they condense at different heights – higher‑boiling fractions condense lower in the column, while lower‑boiling fractions condense higher.

Fractions collected near the top of the column contain short‑chain, small‑molecule hydrocarbons with low boiling points. These molecules are highly flammable and therefore make good fuels. Fractions collected near the bottom contain long‑chain, high‑boiling hydrocarbons that are less flammable and are used for other purposes such as lubricants and base oils.