Crude oil is a finite resource that is separated into useful fractions by fractional distillation. Describe how fractional distillation separates crude oil into fractions, and explain why different fractions are collected at different heights in the fractionating column.

Pearson Edexcel International GCSE Chemistry (4CH1) — 4.2 Crude oil · Describe and Explain · 4 marks · View as Markdown

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

Crude oil is a mixture of hydrocarbons with different boiling points. It is processed at an oil refinery to produce fuels and other useful products.

Model answer (4 marks)

Crude oil is heated until it vapourises.
The vapour rises through a fractionating column that has a temperature gradient – hot at the bottom, cool at the top.
As the vapour cools, hydrocarbons condense when the column temperature reaches their boiling point and are collected as liquid fractions.
Higher‑boiling (longer‑chain) hydrocarbons condense lower in the column, while lower‑boiling (shorter‑chain) hydrocarbons condense higher up or leave as gas at the top.

Examiner tips

  • Use the word ‘vapourises’ and ‘condense’ to show understanding of phase change.
  • Mention the temperature gradient explicitly – hot bottom, cool top.
  • Explain that condensation occurs at each hydrocarbon’s boiling point.
  • Show the link between boiling point and column height (high‑boiling lower, low‑boiling higher).

Common mistakes

  • Confusing the direction of the temperature gradient (e.g. saying the column is cooler at the bottom).
  • Failing to state that condensation occurs at the boiling point of each component.
  • Using vague terms like ‘separated’ without describing the vapour–liquid process.

Mark scheme (4 marks)

  1. Crude oil is heated / vaporised so the hydrocarbons turn into gas / vapour
  2. The vapour rises up the fractionating column, where there is a temperature gradient — hotter at the bottom and cooler at the top
  3. Hydrocarbons condense when the temperature in the column falls to / reaches their boiling point, and are collected as a liquid fraction
  4. Fractions with higher boiling points (longer chain / larger molecules) condense lower down the column, whilst fractions with lower boiling points (shorter chain / smaller molecules) condense higher up / exit as gas at the top

Key terms in this question

fractional distillation · fraction · crude oil · finite resource

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