# Crude oil is a finite resource formed from the remains of ancient organisms. It contains many different hydrocarbons that can be separated by fractional distillation. Explain how fractional distillation separates crude oil into fractions, and describe how the properties of the fractions change from the bottom to the top of the fractionating column.

> Edexcel A-Level Chemistry (9CH0) — 6.1 Introduction to organic chemistry · Explain · 5 marks

> Crude oil is a mixture of hydrocarbon molecules of different chain lengths. It is heated and the vapours rise through a fractionating column, which has a temperature gradient — hottest at the bottom and coolest at the top.

## Mark scheme (5 marks)

1. Crude oil is heated and the hydrocarbons are vaporised
2. The fractionating column has a temperature gradient — hottest at the bottom and coolest at the top
3. Hydrocarbons condense when they reach the part of the column that is at their boiling point / temperature, and are collected as fractions
4. Fractions collected at the bottom have longer carbon chains / higher molecular mass and higher boiling points
5. Fractions collected at the top have shorter carbon chains and are more flammable / more volatile / less viscous than fractions at the bottom

## Key terms

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

## Related

- [Revision notes for Edexcel A-Level Chemistry (9CH0)](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-a-finite-resource-75036a62) · Published by Druglandscape Ltd.