Ethene undergoes an addition reaction with bromine water. Describe what is observed when ethene is bubbled through bromine water, and explain why ethene reacts in this way whilst ethane does not.

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

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

Model answer (4 marks)

Bromine water is decolourised – the orange/brown colour disappears and the solution becomes colourless.

Ethene is unsaturated; it contains a C=C double bond. The double bond opens during the reaction, allowing two bromine atoms to add across it – an addition reaction.

Ethane is saturated; it has only single C–H and C–C bonds. Without a double bond it cannot undergo an addition reaction with bromine, so the bromine water remains coloured.

Examiner tips

  • Use the word "decolourised" to show the colour change.
  • Mention the double bond in ethene and the single bonds in ethane.
  • Explain that addition requires a π bond to break.
  • Keep the answer concise – 4 points total.

Common mistakes

  • Saying the colour changes to green or blue instead of colourless.
  • Confusing ethane with ethene or vice‑versa.
  • Claiming ethane reacts because it has C–H bonds – it needs a C=C bond for addition.

Mark scheme (4 marks)

  1. Bromine water is decolourised (turns from orange/brown to colourless)
  2. Ethene is unsaturated / contains a carbon–carbon double bond
  3. The double bond opens / breaks, allowing bromine to add across it (addition reaction)
  4. Ethane is saturated / has only single bonds, so it cannot undergo an addition reaction with bromine

Key terms in this question

addition reaction · bromine water

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