A student is trying to identify two unknown liquids. One liquid is a haloalkane and the other is an alcohol. The student adds a few drops of each liquid to separate test tubes containing aqueous silver nitrate solution acidified with dilute nitric acid. One test tube produces a cream precipitate, whilst the other shows no change. Explain these observations, identifying which liquid is the haloalkane and which is the alcohol.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Haloalkanes contain halogen atoms bonded to a carbon chain. Alcohols contain an –OH group bonded to a carbon chain. Both compound types are important in organic chemistry.
Model answer (5 marks)
The liquid that gives a cream precipitate is the haloalkane.
When the haloalkane is added to the acidified AgNO₃ solution, the halogen atom (e.g. Br) is displaced as a halide ion:
R–Br + Ag⁺ → R⁺ + Br⁻ + AgBr(s)
The released Br⁻ reacts with Ag⁺ to form the insoluble silver bromide precipitate, which appears cream‑white.
The liquid that shows no change is the alcohol. An –OH group does not liberate halide ions, so no Ag⁺ is consumed and no precipitate forms.
Thus the test distinguishes the haloalkane from the alcohol.
When the haloalkane is added to the acidified AgNO₃ solution, the halogen atom (e.g. Br) is displaced as a halide ion:
R–Br + Ag⁺ → R⁺ + Br⁻ + AgBr(s)
The released Br⁻ reacts with Ag⁺ to form the insoluble silver bromide precipitate, which appears cream‑white.
The liquid that shows no change is the alcohol. An –OH group does not liberate halide ions, so no Ag⁺ is consumed and no precipitate forms.
Thus the test distinguishes the haloalkane from the alcohol.
Examiner tips
- State the precipitate colour and identify the haloalkane first. Explain the ion‑exchange reaction and the formation of AgX. Mention that the alcohol lacks halide ions. Show the logical link: precipitate = haloalkane, no change = alcohol.
Common mistakes
- Confusing the precipitate with a coloured solution; the correct colour is cream‑white. Forgetting to mention that the test uses acidified AgNO₃. Claiming the alcohol reacts, when it actually does not produce halide ions.
Mark scheme (5 marks)
- The haloalkane produces the cream precipitate
- The haloalkane releases / produces halide ions (e.g. bromide ions) in solution
- Silver ions react with halide ions to form a silver halide precipitate
- The alcohol does not react / shows no change because it does not contain / release halide ions
- This test allows the two compounds to be distinguished / identified
Key terms in this question
haloalkane · precipitate · alcohol · silver nitrate
Related
- All OCR A-Level Chemistry A (H432) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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