# Crude oil is separated into fractions by fractional distillation. Describe how the properties of hydrocarbon molecules change as the chain length increases, and explain how these properties affect the suitability of different fractions as fuels.

> AQA A-Level Chemistry (7405) — 3.3.2 Alkanes · Describe · 5 marks

## Mark scheme (5 marks)

1. As chain length increases, boiling point increases (longer molecules have higher boiling points / shorter molecules have lower boiling points)
2. As chain length increases, viscosity increases (longer molecules are more viscous / thicker / less runny)
3. As chain length increases, flammability decreases (shorter molecules are more flammable / ignite more easily)
4. Short-chain fractions are more suitable as fuels because they are more flammable / ignite more easily
5. Short-chain fractions are also more suitable as fuels because they have lower boiling points / vaporise more easily, so they flow and ignite readily (OR: long-chain fractions are less suitable because high viscosity makes them difficult to use as fuels)

## Key terms

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

## Related

- [Revision notes for AQA A-Level Chemistry (7405)](https://www.gradenine.co.uk/learn)
- [How to answer "Describe" 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-47e61980) · Published by Druglandscape Ltd.