# Crude oil is a mixture of hydrocarbons. It is separated into fractions by fractional distillation. One fraction contains octane, C₈H₁₈, which is a member of the alkane homologous series. Explain what is meant by the term 'hydrocarbon' and describe the key features that make alkanes a homologous series, including how their properties change as the chain length increases.

> OCR A-Level Chemistry A (H432) — 4.1 Basic concepts and hydrocarbons · Explain · 5 marks

> Crude oil extracted from beneath the seabed contains hundreds of different hydrocarbon molecules. Refineries separate these molecules to produce fuels and raw materials for the chemical industry.

## Mark scheme (5 marks)

1. A hydrocarbon contains hydrogen and carbon only / is a compound of hydrogen and carbon only
2. Members of a homologous series share the same general formula
3. Each successive member differs by one CH₂ unit
4. Members of the series have similar chemical properties
5. As chain length increases, boiling point increases / they become less flammable / they become more viscous

## Key terms

- [hydrocarbon](https://www.gradenine.co.uk/glossary/hydrocarbon)
- [homologous series](https://www.gradenine.co.uk/glossary/homologous-series)
- [alkane](https://www.gradenine.co.uk/glossary/alkane)

## Related

- [Revision notes for OCR A-Level Chemistry A (H432)](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-ec110153) · Published by Druglandscape Ltd.