# Crude oil contains many hydrocarbons with different chain lengths. Describe how the properties of hydrocarbon molecules change as chain length increases, and explain how these properties affect the usefulness of longer-chain fractions as fuels.

> AQA A-Level Chemistry (7405) — 3.3.10 Aromatic chemistry (A-Level only) · Describe and Explain · 5 marks

## Mark scheme (5 marks)

1. Longer-chain hydrocarbons have a higher boiling point / are harder to vaporise
2. Longer-chain hydrocarbons are more viscous / less runny / flow less easily
3. Longer-chain hydrocarbons are less flammable / do not ignite as easily
4. Because longer-chain fractions are less flammable, they are less useful as fuels (compared to shorter-chain fractions)
5. Cracking can be used to break down longer-chain hydrocarbons into shorter, more useful molecules / shorter-chain alkanes and alkenes

## Key terms

- [hydrocarbon](https://www.gradenine.co.uk/glossary/hydrocarbon)
- [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 "Describe and 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-contains-many-hydrocarbons-with-cebe4d9f) · Published by Druglandscape Ltd.