A student adds bromine water to two test tubes, one containing hexane and one containing ethene. Describe what the student would observe in each test tube and explain why the results are different.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
1. In the tube containing hexane the bromine water remains orange and unchanged.
2. In the tube containing ethene the bromine water turns colourless.
3. Ethene is unsaturated, possessing a carbon–carbon double bond.
4. Hexane is a saturated alkane and does not react with bromine water.
5. The double bond in ethene makes it more reactive; bromine adds across the double bond in an addition reaction, consuming the orange bromine and leaving the solution colourless.
2. In the tube containing ethene the bromine water turns colourless.
3. Ethene is unsaturated, possessing a carbon–carbon double bond.
4. Hexane is a saturated alkane and does not react with bromine water.
5. The double bond in ethene makes it more reactive; bromine adds across the double bond in an addition reaction, consuming the orange bromine and leaving the solution colourless.
Examiner tips
- Use the exact phrases ‘remains orange’ and ‘turns colourless’ to match the mark scheme.
- Explain the difference in terms of saturation and the addition reaction mechanism.
- Keep the answer concise – 5 points, 5 sentences is ideal.
Common mistakes
- Claiming hexane reacts with bromine water; it does not.
- Forgetting to mention the double bond in ethene or the addition mechanism.
- Using UK spelling ‘colourless’ instead of ‘colorless’ (UK spelling is required).
Mark scheme (5 marks)
- Bromine water remains orange/unchanged with hexane
- Bromine water turns colourless with ethene
- Ethene is unsaturated / contains a carbon-carbon double bond
- Hexane is an alkane / saturated hydrocarbon, so does not react with bromine water
- The double bond in ethene makes it more reactive / allows it to react with bromine water (addition reaction / bromine adds across the double bond)
Key terms in this question
Related
- All AQA A-Level Chemistry (7405) revision notes →
- How to answer a "Describe and Explain" question →
- Decode the mark scheme abbreviations →
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