A forensic scientist is analysing an unknown organic liquid recovered from a crime scene. The liquid is suspected to be either methanol or chloromethane. Describe a chemical test the scientist could carry out to identify which compound is present, including the reagents used, the observations expected for each compound, and what those observations indicate.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Methanol is an alcohol. Chloromethane is a haloalkane. Both are colourless liquids at room temperature.
Model answer (5 marks)
Add a few drops of (acidified) silver nitrate solution to a small sample of the unknown liquid.
• If the liquid is chloromethane, a white precipitate of AgCl will form immediately, indicating the presence of chloride ions.
• If the liquid is methanol, no precipitate will appear; the solution remains clear.
The test works because chloride ions from chloromethane react with Ag⁺ to give insoluble AgCl, whereas methanol contains no halide ions and therefore does not react with silver nitrate.
Alternatively, a combustion test can be carried out: a small amount of the liquid is burned in air. If methanol is present, the flame produces water and CO₂ – the water turns cobalt‑chloride paper pink and the CO₂ turns limewater cloudy. Chloromethane gives a different combustion profile, with little or no water produced and a faint CO₂ cloud.
These observations allow the scientist to identify whether the unknown liquid is methanol or chloromethane.
• If the liquid is chloromethane, a white precipitate of AgCl will form immediately, indicating the presence of chloride ions.
• If the liquid is methanol, no precipitate will appear; the solution remains clear.
The test works because chloride ions from chloromethane react with Ag⁺ to give insoluble AgCl, whereas methanol contains no halide ions and therefore does not react with silver nitrate.
Alternatively, a combustion test can be carried out: a small amount of the liquid is burned in air. If methanol is present, the flame produces water and CO₂ – the water turns cobalt‑chloride paper pink and the CO₂ turns limewater cloudy. Chloromethane gives a different combustion profile, with little or no water produced and a faint CO₂ cloud.
These observations allow the scientist to identify whether the unknown liquid is methanol or chloromethane.
Examiner tips
- Use the silver nitrate test first – it gives a clear, quick result. Include the key observation (white precipitate vs none) and explain it. Mention the combustion test as a confirmatory method. Use correct chemical names and symbols (Ag⁺, AgCl, CH₃OH, CH₃Cl).
Common mistakes
- Failing to acidify the silver nitrate solution, giving a false negative. Not recognising that a clear solution means methanol. Confusing the combustion test results or not linking them to the presence of –OH or C–Cl bonds.
Mark scheme (5 marks)
- Add (acidified) silver nitrate solution to the unknown liquid (having first added warm dilute nitric acid / after warming with sodium hydroxide solution for the haloalkane)
- Chloromethane (haloalkane) produces a white precipitate with silver nitrate solution, indicating the presence of a halide / chloride ion
- Methanol (alcohol) produces no precipitate / no change with silver nitrate solution
- Burn / combust a sample and test the combustion products — methanol burns to produce water and carbon dioxide, confirmed by turning cobalt chloride paper pink (water) and turning limewater cloudy (carbon dioxide)
- Methanol is an alcohol so contains an –OH group, whereas chloromethane is a haloalkane containing a C–Cl bond / halogen atom, which is why the silver nitrate test distinguishes them
Related
- All OCR A-Level Chemistry B: Salters (H433) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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