Alkanes and alkenes are both hydrocarbons. Explain how bromine water can be used to distinguish between an alkane and an alkene, and explain why they react differently with bromine water.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A student has two unlabelled test tubes, one containing hexane and one containing hexene. She adds bromine water to each sample.
Model answer (5 marks)
Alkenes decolourise bromine water – the orange/brown colour disappears and the solution becomes colourless. Alkanes do not decolourise – the orange/brown colour remains unchanged.
The reason is that alkenes contain a C=C double bond. The double bond can undergo an addition reaction with Br₂, breaking the double bond and forming a dibromide, which removes the coloured bromine from solution. Alkanes are saturated, having only C–C single bonds, so they cannot undergo addition reactions with bromine water and therefore remain coloured.
The reason is that alkenes contain a C=C double bond. The double bond can undergo an addition reaction with Br₂, breaking the double bond and forming a dibromide, which removes the coloured bromine from solution. Alkanes are saturated, having only C–C single bonds, so they cannot undergo addition reactions with bromine water and therefore remain coloured.
Examiner tips
- State the colour change for each hydrocarbon. Explain that the double bond allows addition of Br₂. Mention that alkanes lack a double bond and cannot add Br₂.
Common mistakes
- Forgetting to mention the colour change of the alkane. Confusing the reaction mechanism (e.g., suggesting substitution instead of addition). Using vague terms like "reacts" without specifying addition or lack of reaction.
Mark scheme (5 marks)
- The alkene decolourises / decolorises the bromine water (from orange/brown to colourless)
- The alkane does not decolourise / does not react with bromine water (stays orange/brown)
- Alkenes contain a carbon–carbon double bond (C=C)
- The double bond in the alkene allows an addition reaction to occur with bromine
- Alkanes are saturated (contain only C–C single bonds) so they cannot undergo addition reactions with bromine water
Key terms in this question
alkane · alkene · bromine water
Related
- All Eduqas A-Level Chemistry revision notes →
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