Propene is a member of the alkene homologous series. Describe the chemical test that would distinguish propene from propane, giving the result observed for each compound.

Pearson Edexcel International GCSE Chemistry (4CH1) — 4.4 Alkenes · Describe · 4 marks · View as Markdown

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

Model answer (4 marks)

Add a few drops of bromine water to a small sample of each compound.
1. Propene – the orange‑brown colour of the bromine water disappears, giving a colourless solution.
2. Propane – the orange‑brown colour remains unchanged.
The colour change occurs because propene contains a C=C double bond that undergoes addition of Br₂, whereas propane is saturated and does not react.

Examiner tips

  • State the test (bromine water) and the observed colour change for each compound.
  • Explain that the reaction is due to the double bond in propene – unsaturation – and that propane is saturated.
  • Use the exact wording "decolourises" or "remains orange‑brown" as the mark scheme expects.
  • Keep the answer concise – 4 marks only.

Common mistakes

  • Confusing the colour change direction (writing that propene remains orange‑brown).
  • Failing to mention that propane does not react with bromine water.
  • Using vague terms like "reacts" without specifying the double bond or the addition reaction.

Mark scheme (4 marks)

  1. Add bromine water to each compound
  2. Propene decolourises / turns colourless the bromine water
  3. Propane does not decolourise the bromine water / bromine water remains orange-brown with propane
  4. Propene reacts because it is unsaturated / has a C=C double bond which undergoes an addition reaction with bromine

Key terms in this question

alkene · homologous series

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