# A student investigates two hydrocarbons: ethane and ethene. Ethene is an unsaturated hydrocarbon, whereas ethane is a saturated hydrocarbon. Explain how the student could use bromine water to distinguish between ethane and ethene, and explain why the two hydrocarbons react differently with bromine water.

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

> Ethane has the molecular formula C₂H₆. Ethene has the molecular formula C₂H₄. Both are hydrocarbons found in the products of cracking long-chain alkanes.

## Mark scheme (5 marks)

1. Add bromine water to each hydrocarbon (or bubble each gas through bromine water)
2. Ethene decolourises / decolorises the bromine water (orange/brown to colourless)
3. Ethane does not decolourise / decolorize the bromine water (stays orange/brown)
4. Ethene is unsaturated and contains a carbon–carbon double bond (C=C)
5. The double bond in ethene opens / breaks, allowing bromine to add across the double bond (addition reaction); ethane has only single bonds so cannot undergo addition

## Key terms

- [hydrocarbon](https://www.gradenine.co.uk/glossary/hydrocarbon)
- [saturated](https://www.gradenine.co.uk/glossary/saturated)
- [unsaturated](https://www.gradenine.co.uk/glossary/unsaturated)
- [bromine water](https://www.gradenine.co.uk/glossary/bromine-water)

## 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/a-student-investigates-two-hydrocarbons-ethane-60339a39) · Published by Druglandscape Ltd.