# Crude oil is a mixture of hydrocarbons. When crude oil is fractionally distilled, it is separated into fractions, each containing molecules with similar chain lengths. Explain why the fractions collected at the top of a fractionating column have lower boiling points than those collected at the bottom, and describe one use for a short-chain hydrocarbon fraction.

> OCR A-Level Chemistry B: Salters (H433) — 4.1 Basic concepts and hydrocarbons · 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 that must be separated before the components can be used as fuels or chemical feedstocks.

## Mark scheme (5 marks)

1. Short-chain hydrocarbons have fewer carbon atoms (and smaller molecules) than long-chain hydrocarbons
2. Short-chain hydrocarbons have weaker intermolecular forces between molecules
3. Less energy is needed to overcome these forces, so boiling point is lower
4. The fractionating column is cooler at the top, so only short-chain (low boiling point) hydrocarbons remain as vapour and are collected there
5. A named use of a short-chain hydrocarbon fraction, e.g. petrol/gasoline used as fuel for cars; LPG used as fuel for cooking/heating; naphtha used as a chemical feedstock

## Key terms

- [hydrocarbon](https://www.gradenine.co.uk/glossary/hydrocarbon)
- [fraction](https://www.gradenine.co.uk/glossary/fraction)
- [boiling point](https://www.gradenine.co.uk/glossary/boiling-point)
- [fractionating column](https://www.gradenine.co.uk/glossary/fractionating-column)

## Related

- [Revision notes for OCR A-Level Chemistry B: Salters (H433)](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-85edd946) · Published by Druglandscape Ltd.