A chemist has three unlabelled bottles. Each bottle contains one of the following liquids: an alkane, an alkene, or a carboxylic acid. Describe how the chemist could use two different chemical tests to identify which bottle contains the alkene and which bottle contains the carboxylic acid. State the expected observations for each test.

AQA A-Level Chemistry (7405) — 3.3.6 Organic analysis · Describe · 5 marks · View as Markdown

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

Model answer (5 marks)

Add bromine water to each sample. The alkene decolourises the orange bromine water to colourless, whereas the alkane and carboxylic acid show no change and remain orange.

Add sodium carbonate (or sodium bicarbonate) to the two remaining samples. The carboxylic acid reacts with the carbonate to give effervescence (CO₂ bubbles), while the alkane shows no reaction.

Examiner tips

  • Use the command word ‘describe’ to outline the steps and observations; include the colour change and effervescence.
  • Mention that the alkane gives no reaction in either test to show it has been excluded.

Common mistakes

  • Failing to state that the alkane shows no change in both tests.
  • Confusing the alkene with the carboxylic acid in the bromine water test.

Mark scheme (5 marks)

  1. Add bromine water to each sample
  2. The alkene decolourises / turns the bromine water from orange to colourless
  3. The alkane and carboxylic acid do not decolourise bromine water (no change / remains orange)
  4. Add sodium carbonate (or sodium hydrogencarbonate) to the remaining two samples to identify the carboxylic acid
  5. The carboxylic acid produces bubbles / effervescence (carbon dioxide gas is produced) with sodium carbonate; the alkane shows no reaction

Key terms in this question

alkene · alkane · carboxylic acid

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