# A student uses bromine water to distinguish between two colourless liquids. One liquid is an alkane and the other is an alkene produced by cracking. Explain how bromine water can be used to identify which liquid is the alkene, and explain why the alkene reacts differently from the alkane.

> AQA A-Level Chemistry (7405) — 3.3.16 Chromatography (A-Level only) · Explain · 5 marks

> Cracking of long-chain hydrocarbons produces smaller alkane and alkene molecules. Alkenes are unsaturated hydrocarbons with the general formula CₙH₂ₙ.

## Mark scheme (5 marks)

1. Add bromine water to each liquid (accept: shake/mix each liquid with bromine water)
2. The liquid that decolourises / turns bromine water from orange to colourless is the alkene
3. The alkane does not decolourise / does not react with bromine water (bromine water stays orange)
4. The alkene has a carbon–carbon double bond (accept: is unsaturated)
5. The double bond makes the alkene more reactive than the alkane, so it reacts with bromine / the alkane is saturated so it does not react 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 AQA A-Level Chemistry (7405)](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/a-student-uses-bromine-water-to-81e20d58) · Published by Druglandscape Ltd.