Hydrocarbons can be described as either saturated or unsaturated. Explain the difference between saturated and unsaturated hydrocarbons, and describe the chemical test that can be used to distinguish between them, including the results you would observe for each type.

Eduqas A-Level Chemistry — 2.5 Hydrocarbons · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

Saturated hydrocarbons contain only single C–C bonds. Unsaturated hydrocarbons contain at least one C=C double bond.

Bromine water is used as a test. When added to an unsaturated hydrocarbon the orange–brown bromine water is decolourised, becoming colourless. With a saturated hydrocarbon the bromine water remains orange‑brown, showing no change.

Examiner tips

  • Use the word "contain" for saturated and "contain at least one" for unsaturated. Mention the bromine water test and the colour change. Keep the answer concise and to the point.

Common mistakes

  • Confusing the colour change direction (saying saturated decolourises).
  • Forgetting to state that bromine water stays orange for saturated hydrocarbons.

Mark scheme (5 marks)

  1. Saturated hydrocarbons contain only single (C–C) bonds between carbon atoms
  2. Unsaturated hydrocarbons contain at least one carbon-carbon double bond (C=C)
  3. Bromine water is added to test for saturation / unsaturation
  4. Unsaturated hydrocarbon decolourises / turns bromine water from orange/brown to colourless
  5. Saturated hydrocarbon does not decolourise bromine water / bromine water stays orange/brown

Key terms in this question

saturated · unsaturated

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