# Crude oil is a mixture of hydrocarbons. Explain how fractional distillation separates crude oil into useful fractions, and state one use of the fraction with the smallest molecules.

> WJEC A-Level Chemistry (Wales) — 2.5 Hydrocarbons · Explain · 5 marks

## Mark scheme (5 marks)

1. Crude oil is heated / vaporised
2. Hydrocarbons have different boiling points
3. The vapour rises up the fractionating column where there is a temperature gradient (hotter at the bottom, cooler at the top)
4. Fractions condense and are collected at different levels / heights in the column
5. A correct use of the fraction with the smallest molecules, e.g. petroleum gas / LPG used as fuel for heating or cooking; petrol used as fuel for cars

## Key terms

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

## Related

- [Revision notes for WJEC A-Level 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-7f99d3d7) · Published by Druglandscape Ltd.