Crude oil is separated into fractions by fractional distillation. Explain why different fractions are collected at different heights in the fractionating column.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Petrol, kerosene, diesel and bitumen are among the fractions obtained from crude oil. Petrol is collected near the top of the fractionating column, whilst bitumen is collected at the bottom.
Model answer (4 marks)
Crude oil is heated and the mixture of hydrocarbons vapourises and rises up the fractionating column.
The column has a temperature gradient – it is hotter at the bottom and cooler at the top.
Fractions with higher boiling points condense lower down the column, while fractions with lower boiling points rise higher before condensing.
Longer‑chain hydrocarbons have higher boiling points, so they condense lower down; shorter‑chain hydrocarbons have lower boiling points and condense higher up.
The column has a temperature gradient – it is hotter at the bottom and cooler at the top.
Fractions with higher boiling points condense lower down the column, while fractions with lower boiling points rise higher before condensing.
Longer‑chain hydrocarbons have higher boiling points, so they condense lower down; shorter‑chain hydrocarbons have lower boiling points and condense higher up.
Examiner tips
- Use the word "condense" to link boiling point to height.
- Mention the temperature gradient explicitly.
- Show the chain‑length/boiling point relationship.
- Keep the answer concise – 4 points only.
Common mistakes
- Confusing the direction of the temperature gradient (saying it is hotter at the top).
- Failing to mention that vapour rises before condensing.
- Using vague terms like "different temperatures" without specifying hot at bottom, cool at top.
Mark scheme (4 marks)
- Crude oil is heated and the mixture of hydrocarbons vaporises / rises up the column
- The column has a temperature gradient — it is hotter at the bottom and cooler at the top
- Fractions with higher boiling points condense lower down / fractions with lower boiling points rise higher before condensing
- Longer-chain hydrocarbons have higher boiling points so condense lower down; shorter-chain hydrocarbons have lower boiling points so condense higher up
Key terms in this question
fractional distillation · fractions · crude oil
Related
- All Pearson Edexcel International GCSE Chemistry (4CH1) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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