Crude oil is separated into useful products using fractional distillation. Explain how fractional distillation works to separate the different fractions in crude oil.

WJEC A-Level Chemistry (Wales) — 1.8 Redox · 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. It is fed into a fractionating column where the different fractions are collected at different levels.

Model answer (5 marks)

Crude oil is heated until it evaporates.
The vapour rises through the fractionating column, where the temperature decreases with height.
Because each hydrocarbon fraction has a distinct boiling point, the vapour condenses back to liquid at the level where the column temperature equals that fraction’s boiling point.
The condensed liquid is then tapped off at that level, giving the different fractions of crude oil.

Examiner tips

  • Use the word "evaporates" for the heating step. Mention the temperature gradient in the column. State that condensation occurs when temperature matches the boiling point. Show the tapping/collection at each level.

Common mistakes

  • Saying the oil is simply "collected" without mentioning condensation. Forgetting to mention the temperature gradient. Using vague terms like "separated" instead of the specific steps of evaporation, condensation and tapping.

Mark scheme (5 marks)

  1. Crude oil is heated so that it evaporates / vaporises
  2. The different hydrocarbons / fractions have different boiling points
  3. The vapour rises up the fractionating column and cools
  4. Each fraction condenses / turns back to liquid at the level where the temperature matches its boiling point
  5. The fractions are tapped off / collected at different levels of the column

Key terms in this question

fractional distillation · fraction

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