A student tests four organic compounds using bromine water and a lit splint. Compound A decolourises bromine water and burns with a clean blue flame. Compound B does not decolourise bromine water and burns with a smoky yellow flame. Compound C does not decolourise bromine water and burns with a clean blue flame. Compound D decolourises bromine water and burns with a smoky yellow flame. The four compounds are ethene, methane, benzene and a long-chain alkene. Identify which compound is methane and which compound is the long-chain alkene. Explain the reasoning behind each identification.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Bromine water is used to distinguish between saturated and unsaturated hydrocarbons. Hydrocarbons with longer carbon chains tend to burn with smokier, less clean flames due to incomplete combustion.
Model answer (5 marks)
Compound C is methane. Methane is a saturated hydrocarbon (alkane) so it does not react with bromine water and therefore does not decolourise it. It has only one carbon atom, giving a low carbon‑to‑hydrogen ratio; this allows complete combustion and a clean blue flame.
Compound D is the long‑chain alkene. An alkene contains a carbon–carbon double bond, so it reacts with bromine water and decolourises it. The long carbon chain gives a high carbon‑to‑hydrogen ratio, which leads to incomplete combustion and a smoky yellow flame.
Compound D is the long‑chain alkene. An alkene contains a carbon–carbon double bond, so it reacts with bromine water and decolourises it. The long carbon chain gives a high carbon‑to‑hydrogen ratio, which leads to incomplete combustion and a smoky yellow flame.
Examiner tips
- Use the exact wording from the mark scheme – e.g. ‘saturated hydrocarbon / alkane’ and ‘unsaturated / has a carbon–carbon double bond’.
- Explain both the bromine test and the flame colour for each compound.
- Show the reasoning for the flame colour in terms of carbon‑to‑hydrogen ratio.
- Keep the answer concise – 5 marks, so one sentence per point.
Mark scheme (5 marks)
- Compound C is methane
- Methane does not decolourise bromine water because it is a saturated hydrocarbon / alkane (no carbon–carbon double bond)
- Methane burns with a clean blue flame because it has a short carbon chain / low carbon-to-hydrogen ratio
- Compound D is the long-chain alkene
- The long-chain alkene decolourises bromine water because it is unsaturated / has a carbon–carbon double bond, AND burns with a smoky yellow flame because the long chain has a high carbon content leading to incomplete combustion
Key terms in this question
Related
- All AQA A-Level Chemistry (7405) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Organic analysis questions
- A chemist has three unlabelled bottles. Each bottle contains one of the followin…
- A student is given four unlabelled liquids. The liquids are ethanol, ethanoic ac…
- A student has three unlabelled bottles. Each bottle contains one of the followin…
- A student has two unlabelled test tubes. One contains an alkane and the other co…