A student is given three unlabelled bottles, each containing a different colourless liquid: chloroethane (a haloalkane), ethanol (an alcohol), and water. Describe how the student could use chemical tests to identify which bottle contains the haloalkane and which contains the alcohol. Include the expected observations for each test.

OCR A-Level Chemistry B: Salters (H433) — 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).

Haloalkanes contain a halogen atom bonded to a carbon chain. Alcohols contain an –OH group. Both groups of compounds have distinct chemical properties that can be used to tell them apart from each other and from water.

Model answer (5 marks)

1. Add a small amount of sodium metal to each bottle.
• Alcohol: vigorous bubbling of H₂ gas (positive test).
• Haloalkane: no reaction (no bubbles).
• Water: no reaction.
2. Take the bottle that gave no bubbles and test for halide ions.
a. Warm the liquid with a dilute NaOH solution to hydrolyse the C–X bond, releasing X⁻.
b. Acidify the solution with dilute HNO₃.
c. Add AgNO₃ solution.
• Haloalkane: white/cream precipitate of AgCl (positive test).
• Alcohol and water: no precipitate.
3. The bottle that gave bubbles is ethanol; the bottle that gave a white precipitate is chloroethane; the bottle that gave neither is water.

Examiner tips

  • Use the sodium test first to separate alcohol from the other two; it gives a clear, observable bubble of H₂. Use the NaOH‑AgNO₃ sequence to confirm the haloalkane; the white precipitate is unmistakable. Remember to label each test clearly and state the expected observation for each bottle. Check that you include the hydrolysis step before adding AgNO₃, as the halide must be released into solution.

Common mistakes

  • Failing to add NaOH before AgNO₃, giving a false negative for the haloalkane. Not recognising that the alcohol gives bubbles with sodium, leading to mis‑identification. Mixing the tests for all three bottles in the same beaker, which can give cross‑contamination of results.

Mark scheme (5 marks)

  1. Add silver nitrate solution (after hydrolysis with sodium hydroxide/warm with water) OR add sodium metal to identify the alcohol — alcohol reacts with sodium to produce bubbles of hydrogen gas
  2. To test for the haloalkane: warm the liquid with sodium hydroxide solution to hydrolyse the C–halogen bond, releasing halide ions into solution
  3. Acidify the resulting solution with dilute nitric acid, then add silver nitrate solution
  4. A white/cream precipitate forms with the haloalkane sample, confirming the presence of halide ions (chloride ions give a white precipitate with silver nitrate)
  5. Water produces no reaction / no precipitate with silver nitrate and no bubbles with sodium, so the bottle that gives neither positive result is identified as water

Key terms in this question

haloalkane · alcohol

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