Crude oil is a mixture of hydrocarbons. Fractional distillation is used to separate crude oil into fractions. Explain how fractional distillation separates crude oil into fractions with different boiling points.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Crude oil contains hydrocarbons with carbon chain lengths ranging from 1 carbon atom to over 70 carbon atoms. Shorter chain hydrocarbons have lower boiling points than longer chain hydrocarbons.
Model answer (4 marks)
Crude oil is heated until most of the hydrocarbons are vapourised.
The vapour rises through a fractionating column that has a temperature gradient – hot at the bottom and cooler at the top.
Hydrocarbons with higher boiling points condense lower in the column and are collected there.
Hydrocarbons with lower boiling points rise higher before condensing and are collected at the top of the column.
The vapour rises through a fractionating column that has a temperature gradient – hot at the bottom and cooler at the top.
Hydrocarbons with higher boiling points condense lower in the column and are collected there.
Hydrocarbons with lower boiling points rise higher before condensing and are collected at the top of the column.
Examiner tips
- Use the word "heated" and "vapourised" to show the initial step. Mention the temperature gradient explicitly. State where high‑boiling and low‑boiling fractions condense. Keep the answer concise – 4 points only.
Common mistakes
- Forgetting to mention the temperature gradient. Confusing the order of collection – saying high‑boiling fractions are collected at the top. Using vague terms like "separated" without explaining the mechanism.
Mark scheme (4 marks)
- Crude oil is heated / vaporised so that most hydrocarbons become gases
- The gases rise up the fractionating column where there is a temperature gradient (hotter at the bottom, cooler at the top)
- Hydrocarbons with higher boiling points condense lower in the column and are collected there
- Hydrocarbons with lower boiling points rise higher before condensing and are collected at the top of the column
Key terms in this question
fractional distillation · hydrocarbons · boiling point · fraction
Related
- All WJEC GCSE Chemistry (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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