# Describe the process of fractional distillation of crude oil and explain how the properties of hydrocarbon fractions are related to the size of their molecules.

> AQA A-Level Chemistry (7405) — 3.3.7 Optical isomerism (A-Level only) · Describe and Explain · 5 marks

## Mark scheme (5 marks)

1. Crude oil is heated and the hydrocarbons evaporate / vaporise
2. Hydrocarbons condense / are collected at different levels / temperatures in the fractionating column because they have different boiling points
3. Each fraction contains molecules with a similar number of carbon atoms
4. Shorter / smaller molecules have lower boiling points / are more volatile / are more flammable
5. Longer / larger molecules have higher viscosity / are less flammable / are less volatile / have higher boiling points

## Key terms

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

## 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)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/describe-the-process-of-fractional-distillation-f0a576f3) · Published by Druglandscape Ltd.