Propanoic acid is a carboxylic acid used as a food preservative. Explain why propanoic acid is described as a weak acid, and state two observable changes that occur when propanoic acid reacts with magnesium.

Cambridge International IGCSE Chemistry (0620) — 11.7 Carboxylic acids · Explain · 4 marks · View as Markdown

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

Propanoic acid, CH₃CH₂COOH, is a carboxylic acid that can react with metals such as magnesium.

Model answer (4 marks)

Propanoic acid is a weak acid because it is only partially ionised in aqueous solution, producing a lower concentration of H⁺ ions than a strong acid of the same concentration.

When propanoic acid reacts with magnesium two observable changes occur:
1. Bubbles of hydrogen gas are produced (effervescence).
2. The magnesium metal dissolves or disappears.

Examiner tips

  • Use the term ‘partially ionised’ or ‘incompletely dissociated’ to show understanding of weak acid behaviour.
  • Mention the lower H⁺ concentration compared with a strong acid of the same molarity.
  • Describe the effervescence and the dissolution of magnesium as the two observable changes.
  • Keep the answer concise and use correct chemical terminology.

Common mistakes

  • Saying the acid is ‘strong’ or ‘completely ionised’ – opposite of the question.
  • Failing to mention the lower H⁺ concentration or the dissolution of magnesium.
  • Using vague terms like ‘reacts’ without specifying the observable changes.

Mark scheme (4 marks)

  1. Propanoic acid is partially / incompletely ionised (dissociated) in aqueous solution
  2. It produces a lower concentration of H⁺ ions than a strong acid of the same concentration
  3. Bubbles / effervescence are produced (hydrogen gas is given off)
  4. The magnesium dissolves / disappears

Key terms in this question

weak acid · carboxylic acid · propanoic acid

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