A student uses bromine water to distinguish between two colourless liquids. One liquid is an alkane and the other is an alkene produced by cracking. Explain how bromine water can be used to identify which liquid is the alkene, and explain why the alkene reacts differently from the alkane.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Cracking of long-chain hydrocarbons produces smaller alkane and alkene molecules. Alkenes are unsaturated hydrocarbons with the general formula CₙH₂ₙ.
Model answer (5 marks)
Add bromine water to each liquid (shake/mix). The liquid that decolourises the orange bromine water to colourless is the alkene. The other liquid (alkane) remains orange as it does not react. The alkene contains a carbon–carbon double bond, making it unsaturated and more reactive, so it adds bromine across the double bond. The alkane is saturated and does not react with bromine water.
Examiner tips
- Mention decolourisation of bromine water as key evidence
- Explain unsaturation of alkene as reason for reactivity
Common mistakes
- Confusing colour change direction (colourless to orange)
- Claiming alkane reacts with bromine water
Mark scheme (5 marks)
- Add bromine water to each liquid (accept: shake/mix each liquid with bromine water)
- The liquid that decolourises / turns bromine water from orange to colourless is the alkene
- The alkane does not decolourise / does not react with bromine water (bromine water stays orange)
- The alkene has a carbon–carbon double bond (accept: is unsaturated)
- The double bond makes the alkene more reactive than the alkane, so it reacts with bromine / the alkane is saturated so it does not react with bromine water
Key terms in this question
alkene · alkane · bromine water
Related
- All AQA A-Level Chemistry (7405) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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