# Alkenes are a homologous series of hydrocarbons. Explain how bromine water can be used to distinguish between an alkane and an alkene, and explain why alkenes undergo this reaction more readily than alkanes.

> Edexcel A-Level Chemistry (9CH0) — 6.3 Alkenes · Explain · 5 marks

> Alkenes contain a carbon-carbon double bond (C=C). Alkanes contain only carbon-carbon single bonds. Both are hydrocarbons.

## Mark scheme (5 marks)

1. Bromine water changes from orange/brown to colourless with the alkene
2. Bromine water remains orange/brown with the alkane (no reaction)
3. The alkene reacts with bromine water because it is unsaturated / contains a C=C double bond
4. The alkene undergoes an addition reaction where bromine adds across the double bond
5. The alkane is saturated / has only single bonds and so cannot undergo addition reactions with bromine water

## Key terms

- [alkene](https://www.gradenine.co.uk/glossary/alkene)
- [alkane](https://www.gradenine.co.uk/glossary/alkane)
- [bromine water](https://www.gradenine.co.uk/glossary/bromine-water)

## 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/alkenes-are-a-homologous-series-of-31a5aa1f) · Published by Druglandscape Ltd.