Explain why ethene is described as an unsaturated hydrocarbon and describe a chemical test that confirms this property.

Edexcel GCSE Chemistry (1CH0) — 10.2 Alkenes (Chem only) · Explain · 4 marks · View as Markdown

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

Ethene is a small molecule produced during the cracking of long-chain hydrocarbons. It is widely used in the polymer industry.

Model answer (4 marks)

Ethene is an unsaturated hydrocarbon because it contains a carbon‑to‑carbon double bond (C=C). Unsaturated means the molecule has fewer hydrogen atoms than the corresponding alkane, so it is not fully saturated with hydrogen.

A chemical test that confirms this is the bromine water test. When bromine water (Br₂ in water) is added to ethene, the orange‑brown colour of the bromine disappears and the solution becomes colourless, indicating addition of Br₂ across the C=C bond.

Examiner tips

  • Use the term "unsaturated" to explain the lack of maximum hydrogen atoms.
  • Mention the C=C double bond explicitly.
  • Describe the bromine water test and the colour change.
  • Show the reaction as a simple addition of Br₂ to the double bond.

Common mistakes

  • Failing to state that ethene has a C=C bond.
  • Confusing saturated with unsaturated or vice versa.
  • Not mentioning the colour change from orange‑brown to colourless.

Mark scheme (4 marks)

  1. Ethene contains a carbon-to-carbon double bond (C=C)
  2. Unsaturated means it does not contain the maximum number of hydrogen atoms / has fewer hydrogen atoms than the equivalent alkane
  3. Add bromine water to ethene
  4. Bromine water decolourises / turns from orange-brown to colourless, confirming the presence of a C=C bond

Key terms in this question

unsaturated · hydrocarbon

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