# Crude oil is a mixture of hydrocarbons. Fractional distillation is used to separate crude oil into fractions, each of which contains hydrocarbons with similar chain lengths. Explain how fractional distillation separates crude oil into fractions, and describe how the properties of the fractions change as the chain length of the hydrocarbons increases.

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

> Crude oil is a finite resource formed over millions of years from the remains of ancient marine organisms. It is a complex mixture of hydrocarbon molecules of different chain lengths.

## Mark scheme (5 marks)

1. Crude oil is heated / vaporised so the hydrocarbons evaporate
2. Hydrocarbons with different chain lengths have different boiling points
3. Shorter-chain / lower boiling point hydrocarbons rise higher up the fractionating column and condense / are collected at the top
4. As chain length increases, boiling point increases
5. As chain length increases, viscosity increases AND/OR flammability (ease of ignition) decreases

## Key terms

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

## 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)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/crude-oil-is-a-mixture-of-d9ec20c5) · Published by Druglandscape Ltd.