Petroleum is a mixture of hydrocarbons that is separated into fractions by fractional distillation. Explain how fractional distillation separates petroleum into fractions, and state one use of the fraction that contains the shortest hydrocarbon molecules.

Cambridge International IGCSE Chemistry (0620) — 11.3 Fuels · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Petroleum is heated until it vapourises and the vapour enters a fractionating column. The column has a temperature gradient – hot at the bottom, cool at the top. As the vapour rises, hydrocarbons condense at the height where the temperature equals their boiling point. Short‑chain, small molecules have lower boiling points and therefore condense higher up the column, while larger molecules condense lower down. The fraction that contains the shortest hydrocarbon molecules is refinery gas (LPG), which is used as a fuel for cooking, heating and other domestic or industrial applications.

Examiner tips

  • Use the word ‘fractionating column’ and mention the temperature gradient; link boiling point to condensation height.
  • Identify the shortest‑chain fraction as refinery gas (LPG) and give one specific use such as cooking or heating.
  • Show the sequence: heating → vapour → column → condensation at boiling point.
  • Keep the answer concise – 4 marks only.

Common mistakes

  • Failing to mention the temperature gradient in the column.
  • Confusing the fraction with the longest chains (diesel) instead of the shortest (LPG).
  • Giving a use that is not for the shortest‑chain fraction, e.g. using diesel as the answer.

Mark scheme (4 marks)

  1. Petroleum is heated / vaporised and the vapours enter the fractionating column
  2. The column has a temperature gradient — hotter at the bottom and cooler at the top
  3. Hydrocarbons condense / are collected at the level where the temperature equals their boiling point; shorter-chain / smaller hydrocarbons have lower boiling points and condense / collect higher up the column
  4. The fraction with the shortest molecules is used as a fuel for cooking / heating / LPG / refinery gas / Calor gas (accept: used as a fuel in general if shortest-chain fraction is correctly identified as refinery/LPG gas)

Key terms in this question

petroleum · fractional distillation · fraction · hydrocarbon

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