A student has two unlabelled test tubes. One contains an alkane and the other contains an alkene. Describe how the student could use bromine water to identify which test tube contains the alkene. Include the observations expected for both test tubes.

AQA A-Level Chemistry (7405) — 3.3.6 Organic analysis · Describe · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (5 marks)

Add a small amount of bromine water to each test tube.
1. In the test tube containing the alkene the orange bromine water decolourises, becoming colourless.
2. In the test tube containing the alkane the orange colour remains unchanged.
3. The alkene reacts with bromine water because it has a carbon–carbon double bond.
4. The alkane does not react because it is saturated and has no double bond.

Examiner tips

  • Use the command word ‘describe’ – give a clear, step‑by‑step procedure. Show the expected colour change for each tube. Mention the reason for the reaction (double bond) and lack of reaction (saturation).

Common mistakes

  • Mixing the two samples together before adding bromine water. Failing to mention that the alkane remains orange. Confusing the colour change with a different reagent.

Mark scheme (5 marks)

  1. Add bromine water to each test tube / add bromine water to both samples
  2. The alkene decolourises / turns the bromine water from orange to colourless
  3. The alkane does not decolourise the bromine water / bromine water stays orange with the alkane
  4. The alkene reacts with bromine water because it is unsaturated / has a carbon–carbon double bond
  5. The alkane does not react with bromine water because it is saturated / has no double bond

Key terms in this question

alkane · alkene · bromine water

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