Ethene and ethane are both hydrocarbons containing two carbon atoms. Explain why ethene is described as an unsaturated hydrocarbon and describe one chemical test that distinguishes ethene from ethane, including the expected results for each compound.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Alkenes such as ethene are produced during the cracking of longer-chain hydrocarbons from crude oil. Unlike the alkanes produced alongside them, alkenes have different chemical properties due to their structure.
Model answer (5 marks)
Ethene is unsaturated because it contains a carbon–carbon double bond (C=C). Ethane is saturated because it contains only single bonds and no double bonds.
A common test is the bromine water test. Bromine water is orange–brown. When ethene is added, the double bond reacts with Br₂, decolourising the solution to colourless. With ethane, no reaction occurs and the orange–brown colour remains.
A common test is the bromine water test. Bromine water is orange–brown. When ethene is added, the double bond reacts with Br₂, decolourising the solution to colourless. With ethane, no reaction occurs and the orange–brown colour remains.
Examiner tips
- State the unsaturation of ethene and saturation of ethane explicitly. Mention the bromine water test and the colour change. Keep the answer concise and use correct chemical terminology.
Common mistakes
- Confusing the colour change direction (colourless → orange) or omitting the test. Using the wrong test (e.g., hydrogenation) instead of bromine water. Failing to mention that ethane does not react.
Mark scheme (5 marks)
- Ethene is unsaturated because it contains a carbon–carbon double bond (C=C)
- Ethane is saturated because it contains only single bonds / no double bonds
- The test uses bromine water
- Ethene decolourises / turns the bromine water from orange/brown to colourless
- Ethane does not decolourise the bromine water / bromine water stays orange/brown with ethane
Key terms in this question
unsaturated · saturated · hydrocarbon
Related
- All OCR A-Level Chemistry B: Salters (H433) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →