A student wants to separate crude oil into its different fractions. Explain how fractional distillation can be used to do this.

WJEC A-Level Chemistry (Wales) — 1.5 Energetics · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Crude oil is a mixture of many hydrocarbons with different boiling points. Fractional distillation is used in industry to separate crude oil into useful fractions.

Model answer (5 marks)

Crude oil is heated so that it evaporates. The hydrocarbons in the mixture have different boiling points. The vapour rises up the fractionating column and condenses at different temperatures along the column. Each fraction is tapped off at a different level of the column. Each fraction therefore contains molecules with a similar number of carbon atoms and similar boiling points.

Examiner tips

Use the word "evaporates" or "vaporises" to show understanding of heating. Mention the fractionating column and that vapour condenses at different temperatures. State that fractions are collected at different levels. Highlight that fractions have similar boiling points/number of carbon atoms.

Common mistakes

Failing to mention the fractionating column or that vapour condenses. Using vague terms like "separate by boiling point" without explaining the column. Confusing the order of steps or omitting that each fraction is tapped off.

Mark scheme (5 marks)

Crude oil is heated / the mixture is heated so that it evaporates / vaporises
The hydrocarbons / fractions have different boiling points
The vapour rises up the fractionating column and condenses at different levels / temperatures
Each fraction is tapped off / collected at a different level of the fractionating column
Each fraction contains molecules with a similar number of carbon atoms / similar boiling points

Key terms in this question

fractional distillation · fraction