Alkanes and alkenes are both hydrocarbons. Describe how you could distinguish between hexane and hex-1-ene using a chemical test, and explain what the results of this test show about the bonding in each compound.

Eduqas GCSE Chemistry — C7 Organic chemistry · 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 sample of each compound.
1. Hex‑1‑ene: the orange/brown colour of the bromine water disappears, becoming colourless.
2. Hexane: the orange/brown colour remains unchanged.
The decolourisation of the bromine water by hex‑1‑ene shows that the C=C double bond reacts with bromine (bromination), breaking the double bond. Hexane, which contains only C–C single bonds, does not react with bromine, so the colour persists.

Examiner tips

  • Use the word ‘decolourises’ or ‘bleaches’ to show the reaction. Mention that bromine water is orange/brown. Explain that the reaction occurs only with C=C bonds. Show the two separate observations clearly.

Common mistakes

  • Not specifying that bromine water is orange/brown. Saying hexane reacts with bromine. Using vague terms like ‘reacts’ without linking to double bonds.

Mark scheme (4 marks)

  1. Add bromine water to each compound
  2. Hex-1-ene decolourises / bleaches the bromine water (orange/brown to colourless)
  3. Hexane does not decolourise the bromine water / bromine water stays orange/brown with hexane
  4. Hex-1-ene contains a carbon–carbon double bond (C=C) which reacts with / is broken by bromine, whereas hexane only has single bonds and does not react

Key terms in this question

alkane · alkene

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