Explain why alkenes react with bromine water but alkanes do not, and describe what you would observe when bromine water is added to an alkene.

AQA A-Level Chemistry (7405) — 3.3.4 Alkenes · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

Alkenes contain a carbon–carbon double bond, making them unsaturated and more reactive than alkanes, which are saturated and contain only single bonds. The double bond undergoes electrophilic addition with Br₂, so bromine water (orange) turns colourless when added to an alkene. With an alkane the double bond is absent, so bromine water remains orange and unchanged.

Examiner tips

  • Mention the double bond and unsaturation first
  • Explain electrophilic addition to give colour change
  • State that alkanes lack a double bond and stay orange
  • Use exact terms: unsaturated, saturated, electrophilic addition

Common mistakes

  • Confusing alkanes with alkenes in the explanation
  • Failing to state that bromine water turns colourless with an alkene
  • Using vague terms like 'reactive' without linking to the double bond

Mark scheme (5 marks)

  1. Alkenes are unsaturated / contain a carbon-carbon double bond
  2. The double bond makes alkenes more reactive than alkanes
  3. Alkanes are saturated / contain only single bonds / no double bond
  4. Bromine water turns from orange to colourless with an alkene
  5. Bromine water remains orange / unchanged with an alkane

Key terms in this question

alkene · bromine water · alkane

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