Ethane (C₂H₆) and ethene (C₂H₄) are both hydrocarbons. Ethene is described as an unsaturated hydrocarbon, while ethane is described as a saturated hydrocarbon. Explain the difference between a saturated and an unsaturated hydrocarbon, and describe a chemical test that would allow you to distinguish between ethane and ethene, including the result for each compound.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Ethane and ethene are both colourless gases at room temperature and are members of different homologous series.
Model answer (4 marks)
A saturated hydrocarbon contains only single C–C bonds. An unsaturated hydrocarbon contains at least one C=C double bond.
Add a few drops of bromine water to each gas.
• With ethene the bromine water decolourises (orange‑brown → colourless) because the double bond reacts.
• With ethane the bromine water remains orange‑brown, showing no reaction.
Add a few drops of bromine water to each gas.
• With ethene the bromine water decolourises (orange‑brown → colourless) because the double bond reacts.
• With ethane the bromine water remains orange‑brown, showing no reaction.
Examiner tips
- Use the exact terms ‘saturated’ and ‘unsaturated’.
- State the reaction of bromine water clearly for each compound.
- Show the colour change for ethene and the lack of change for ethane.
- Keep the answer concise – 4 marks only.
Common mistakes
- Confusing the colour change direction (colourless → orange‑brown).
- Forgetting to mention that ethane is a saturated hydrocarbon.
- Using vague terms like ‘reactive’ instead of ‘double bond’.”
Mark scheme (4 marks)
- A saturated hydrocarbon contains only single carbon–carbon bonds (C–C).
- An unsaturated hydrocarbon contains at least one carbon–carbon double bond (C=C).
- Add bromine water to each compound — the bromine water decolourises / turns from orange-brown to colourless with ethene.
- The bromine water remains orange-brown / does not decolourise with ethane.
Key terms in this question
saturated · unsaturated · hydrocarbon
Related
- All WJEC GCSE Chemistry (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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