# A student has three unlabelled bottles. Each bottle contains one of the following substances: ethanoic acid, ethene and butane. The student adds a sample from each bottle to separate tubes of bromine water. Only one sample decolourises the bromine water. The student then tests the remaining two samples with universal indicator solution. Explain how the results of both tests allow the student to identify all three substances.

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

## Mark scheme (5 marks)

1. The sample that decolourises bromine water is ethene / the alkene
2. Ethene / alkenes decolourise bromine water because they are unsaturated / have a carbon–carbon double bond (that reacts with bromine)
3. The sample that turns universal indicator acidic / red-orange / gives a low pH is ethanoic acid
4. The remaining sample that does not change / leaves universal indicator green / neutral is butane
5. Butane does not decolourise bromine water because it is saturated / has no double bond / is an alkane

## Key terms

- [bromine water](https://www.gradenine.co.uk/glossary/bromine-water)
- [decolourise](https://www.gradenine.co.uk/glossary/decolourise)
- [universal indicator](https://www.gradenine.co.uk/glossary/universal-indicator)

## 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-has-three-unlabelled-bottles-366254b8) · Published by Druglandscape Ltd.