# 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.

> Eduqas GCSE Chemistry — 2.6 Carbon chemistry / organic chemistry · Explain · 4 marks

> Crude oil is a fossil fuel formed over millions of years from the remains of marine organisms. It contains hydrocarbons of different chain lengths.

## Mark scheme (4 marks)

1. The crude oil is heated and the hydrocarbons vaporise / turn to gas
2. Hydrocarbons with shorter carbon chains / smaller molecules have lower boiling points and rise higher up the column
3. There is a temperature gradient in the column — it is cooler at the top and hotter at the bottom
4. Hydrocarbons condense / turn back to liquid when they reach the part of the column that is at their boiling point, and are collected as separate fractions

## Key terms

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

## Related

- [Revision notes for Eduqas GCSE Chemistry](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/crude-oil-is-a-mixture-of-32113b1a) · Published by Druglandscape Ltd.