A forensic scientist is analysing an unknown organic liquid found at a crime scene. The scientist suspects the liquid is either a haloalkane or an alcohol. Describe the chemical tests the scientist could carry out to distinguish between a haloalkane and an alcohol, including the expected observations for each compound.

OCR A-Level Chemistry A (H432) — 4.2 Alcohols, haloalkanes and analysis · Describe · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Forensic scientists routinely use chemical tests to identify unknown organic compounds. Two common classes of organic compound are alcohols, which contain the –OH functional group, and haloalkanes, which contain a halogen atom bonded to a carbon chain.

Model answer (5 marks)

1. Add a few drops of the liquid to water and then add a small piece of sodium metal (or state the reaction with sodium).
2. If the liquid is an alcohol, the sodium reacts vigorously, giving effervescence of hydrogen gas:


2 Na + 2 ROH → 2 NaOH + R–H + H₂↑

The bubbling is a clear sign of an alcohol.
3. If the liquid is a haloalkane, sodium does not react (or reacts very slowly) and no bubbles are seen.
4. To determine whether the liquid is a haloalkane and to identify the halogen, heat the liquid with aqueous NaOH (hydrolysis) and then acidify with dilute HNO₃.
5. Add AgNO₃ solution.
• A white precipitate of AgCl indicates a chloro‑haloalkane.
• A cream‑yellow precipitate of AgBr indicates a bromo‑haloalkane.
• A yellow precipitate of AgI indicates an iodo‑haloalkane.
6. If no precipitate forms after the acidification and AgNO₃ test, the liquid is not a haloalkane (hence it is an alcohol).

Examiner tips

  • Use the sodium test first – the presence of bubbles gives instant evidence of an alcohol.
  • Show the hydrolysis and acidification steps before adding AgNO₃; this sequence is key to the halogen‑identification test.
  • Mention the colour of the precipitate to match the specific halogen.
  • Keep the answer concise and use the exact terminology (e.g., effervescence, precipitate).

Common mistakes

  • Failing to state the sodium reaction or the effervescence for alcohols.
  • Not recognising that no reaction with sodium indicates a haloalkane.
  • Mixing up the colours of the AgNO₃ precipitates (e.g., calling AgBr cream instead of yellow).

Mark scheme (5 marks)

  1. Add the unknown liquid to water and then add sodium (or state reaction with sodium metal) — alcohol reacts with sodium to produce bubbles of hydrogen gas / effervescence
  2. Haloalkane does not react (or reacts very slowly / no bubbles) with sodium compared to an alcohol
  3. To identify the halogen in the haloalkane: heat the haloalkane with sodium hydroxide solution (to hydrolyse it) then acidify with dilute nitric acid and add silver nitrate solution
  4. A precipitate forms with the haloalkane: white precipitate indicates chloroalkane, cream precipitate indicates bromoalkane, yellow precipitate indicates iodoalkane
  5. No precipitate forms when the silver nitrate test is applied to the alcohol (after the same acidification step), confirming it is not a haloalkane

Key terms in this question

haloalkane · alcohol

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