# Crude oil is a mixture of hydrocarbons. Fractional distillation is used to separate crude oil into fractions. Explain how fractional distillation separates crude oil into fractions, and describe one property that differs between a fraction collected near the top of the fractionating column and a fraction collected near the bottom.

> Eduqas A-Level Chemistry — 2.4 Organic compounds · Explain · 5 marks

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

## Mark scheme (5 marks)

1. Crude oil is heated / vaporised and the vapours enter the fractionating column
2. Different hydrocarbons have different boiling points
3. Hydrocarbons condense when the temperature in the column falls to their boiling point / condense at different levels
4. The column is hotter at the bottom and cooler at the top, so fractions with higher boiling points are collected lower down
5. A named property that correctly differs between a top fraction and a bottom fraction — e.g. fractions near the top have shorter carbon chains / lower boiling points / are more flammable / less viscous compared to fractions near the bottom which have longer carbon chains / higher boiling points / are less flammable / more viscous

## Key terms

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

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- [Revision notes for Eduqas A-Level Chemistry](https://www.gradenine.co.uk/learn)
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Source: [GradeNine](https://www.gradenine.co.uk/q/crude-oil-is-a-mixture-of-f94d39d0) · Published by Druglandscape Ltd.