# Crude oil is separated into fractions by fractional distillation. Each fraction contains hydrocarbons with different chain lengths. Explain why shorter-chain hydrocarbons are more useful as fuels than longer-chain hydrocarbons.

> WJEC A-Level Chemistry (Wales) — 3.2 Hydrocarbons · Explain · 5 marks

> Crude oil contains hydrocarbons ranging from very short carbon chains to very long carbon chains. Short-chain hydrocarbons are found in fractions collected near the top of the fractionating column, while long-chain hydrocarbons are collected near the bottom.

## Mark scheme (5 marks)

1. Short-chain hydrocarbons have lower boiling points (than long-chain hydrocarbons)
2. Short-chain hydrocarbons are more flammable / ignite more easily
3. Short-chain hydrocarbons flow more easily / have lower viscosity
4. Short-chain hydrocarbons are gases or liquids at room temperature, making them easier to store / transport / use as fuels
5. Long-chain hydrocarbons are more difficult to burn completely / produce more incomplete combustion / more soot

## Key terms

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

## 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)
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Source: [GradeNine](https://www.gradenine.co.uk/q/crude-oil-is-separated-into-fractions-34d0063d) · Published by Druglandscape Ltd.