# A student tests four organic compounds using bromine water and a lit splint. Compound A decolourises bromine water and burns with a clean blue flame. Compound B does not decolourise bromine water and burns with a smoky yellow flame. Compound C does not decolourise bromine water and burns with a clean blue flame. Compound D decolourises bromine water and burns with a smoky yellow flame. The four compounds are ethene, methane, benzene and a long-chain alkene. Identify which compound is methane and which compound is the long-chain alkene. Explain the reasoning behind each identification.

> AQA A-Level Chemistry (7405) — 3.3.6 Organic analysis · Explain · 5 marks

> Bromine water is used to distinguish between saturated and unsaturated hydrocarbons. Hydrocarbons with longer carbon chains tend to burn with smokier, less clean flames due to incomplete combustion.

## Mark scheme (5 marks)

1. Compound C is methane
2. Methane does not decolourise bromine water because it is a saturated hydrocarbon / alkane (no carbon–carbon double bond)
3. Methane burns with a clean blue flame because it has a short carbon chain / low carbon-to-hydrogen ratio
4. Compound D is the long-chain alkene
5. The long-chain alkene decolourises bromine water because it is unsaturated / has a carbon–carbon double bond, AND burns with a smoky yellow flame because the long chain has a high carbon content leading to incomplete combustion

## Key terms

- [decolourises](https://www.gradenine.co.uk/glossary/decolourises)
- [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-tests-four-organic-compounds-2903f8b8) · Published by Druglandscape Ltd.