# Crude oil is separated by fractional distillation. Explain how the properties of hydrocarbon molecules affect their separation into fractions and their usefulness as fuels.

> AQA A-Level Chemistry (7405) — 3.3.1 Introduction to organic chemistry · Explain · 5 marks

## Mark scheme (5 marks)

1. Crude oil is a mixture of hydrocarbons with different (chain) lengths / different numbers of carbon atoms
2. Hydrocarbons with shorter chains / smaller molecules have lower boiling points and condense / are collected higher up the column
3. Hydrocarbons evaporate and rise up the fractionating column, condensing when they reach the level at the temperature equal to their boiling point, allowing separation into fractions
4. Shorter hydrocarbon molecules are more flammable / ignite more easily, making them more useful as fuels
5. Shorter hydrocarbon molecules are less viscous / flow more easily (than longer chain molecules)

## Key terms

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

## Related

- [Revision notes for AQA A-Level Chemistry (7405)](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-separated-by-fractional-b1d848c2) · Published by Druglandscape Ltd.