# 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.

> WJEC GCSE Chemistry (Wales) — 2.6 Carbon chemistry / organic chemistry · Explain · 4 marks

> 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.

## Mark scheme (4 marks)

1. Crude oil is heated / vaporised so that most hydrocarbons become gases
2. The gases rise up the fractionating column where there is a temperature gradient (hotter at the bottom, cooler at the top)
3. Hydrocarbons with higher boiling points condense lower in the column and are collected there
4. Hydrocarbons with lower boiling points rise higher before condensing and are collected at the top of the column

## Key terms

- [fractional distillation](https://www.gradenine.co.uk/glossary/fractional-distillation)
- [hydrocarbons](https://www.gradenine.co.uk/glossary/hydrocarbons)
- [boiling point](https://www.gradenine.co.uk/glossary/boiling-point)
- [fraction](https://www.gradenine.co.uk/glossary/fraction)

## Related

- [Revision notes for WJEC GCSE Chemistry (Wales)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/crude-oil-is-a-mixture-of-e19b2cf2) · Published by Druglandscape Ltd.