# Crude oil is a mixture of hydrocarbons. Fractional distillation is used to separate crude oil into fractions, and some fractions are then cracked to produce more useful products. Explain how fractional distillation separates crude oil into fractions, and state one reason why cracking is carried out.

> Edexcel A-Level Chemistry (9CH0) — Topic 5 Organic chemistry II · Explain · 5 marks

> Crude oil is a finite resource formed over millions of years from the remains of ancient organisms. It is the source of many important fuels and chemical feedstocks.

## Mark scheme (5 marks)

1. Crude oil is heated / vaporised so the hydrocarbons evaporate
2. Different hydrocarbons / fractions have different boiling points
3. Hydrocarbons condense / are collected at different levels / temperatures in the fractionating column
4. Longer chain hydrocarbons have higher boiling points / condense lower in the column; shorter chain hydrocarbons have lower boiling points / condense higher in the column
5. Cracking is carried out because there is greater demand for shorter chain / lighter fractions (such as petrol / fuels) than for longer chain / heavier fractions, OR to produce alkenes which can be used as monomers / in making plastics

## Key terms

- [fractional distillation](https://www.gradenine.co.uk/glossary/fractional-distillation)
- [fraction](https://www.gradenine.co.uk/glossary/fraction)
- [cracking](https://www.gradenine.co.uk/glossary/cracking)
- [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-mixture-of-65038259) · Published by Druglandscape Ltd.