A student tests an unknown organic liquid using a variety of chemical tests. The liquid does not decolourise bromine water, gives a silver mirror with Tollens' reagent, and produces a cream precipitate when tested with acidified silver nitrate solution. Explain what each of these results tells the student about the functional groups present in the unknown liquid.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
An unknown organic liquid is subjected to three chemical tests: bromine water, Tollens' reagent, and acidified silver nitrate solution.
Model answer (5 marks)
No decolourisation of bromine water shows that the liquid contains no C=C double bond – it is not an alkene or other unsaturated compound.
A silver mirror with Tollens' reagent indicates the presence of an aldehyde group (–CHO). The aldehyde is oxidised and reduces Ag⁺ to metallic silver.
A cream precipitate with acidified silver nitrate demonstrates that bromide ions are present, meaning the liquid contains a C–Br bond (a bromoalkane). The precipitate is silver bromide (AgBr).
A silver mirror with Tollens' reagent indicates the presence of an aldehyde group (–CHO). The aldehyde is oxidised and reduces Ag⁺ to metallic silver.
A cream precipitate with acidified silver nitrate demonstrates that bromide ions are present, meaning the liquid contains a C–Br bond (a bromoalkane). The precipitate is silver bromide (AgBr).
Examiner tips
- Use the exact terminology: "no C=C double bond", "aldehyde (–CHO)", "silver bromide (AgBr)". Show the logical link between each test and the functional group. Keep the answer concise – one sentence per test is sufficient for 5 marks.
Common mistakes
- Claiming the liquid is an alkene when bromine is not decolourised. Confusing the silver mirror with a reduction of any alcohol instead of specifying an aldehyde. Forgetting to state that the cream precipitate is AgBr or that it indicates a C–Br bond.
Mark scheme (5 marks)
- No decolourisation of bromine water shows there is no C=C double bond (no alkene / unsaturation) present
- Silver mirror with Tollens' reagent shows an aldehyde group (–CHO) is present
- Tollens' reagent works because the aldehyde reduces the silver ions to silver metal / the aldehyde is oxidised
- Cream precipitate with acidified silver nitrate shows a bromide ion (Br⁻) is present / the liquid contains a C–Br bond (bromoalkane)
- The cream precipitate is silver bromide (AgBr)
Key terms in this question
bromine water · Tollens' reagent · acidified silver nitrate
Related
- All Eduqas A-Level Chemistry revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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