# 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, and state one use for the fraction with the smallest molecules.

> WJEC A-Level Chemistry (Wales) — 2.4 Organic compounds · Explain · 5 marks

> Crude oil is a finite resource formed over millions of years from the remains of ancient organisms. It is a complex mixture of hydrocarbon compounds.

## Mark scheme (5 marks)

1. Crude oil is heated / vaporised so the hydrocarbons evaporate
2. Hydrocarbons have different boiling points
3. Hydrocarbons with smaller/shorter molecules have lower boiling points and rise higher up the column / condense higher up the column
4. Hydrocarbons condense / turn back to liquid when the temperature in the column equals their boiling point, and are collected as separate fractions
5. A correct use for the fraction with the smallest molecules (e.g. refinery gases / LPG used as fuel for heating or cooking / bottled gas)

## Key terms

- [fractional distillation](https://www.gradenine.co.uk/glossary/fractional-distillation)
- [hydrocarbon](https://www.gradenine.co.uk/glossary/hydrocarbon)
- [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-e0e6f507) · Published by Druglandscape Ltd.