Describe the characteristic reactions of a base with an acid and with ammonium salts, stating what is observed in each reaction.

Cambridge International IGCSE Chemistry (0620) — 7.1 The characteristic properties of acids and bases · Describe · 4 marks · View as Markdown

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

Model answer (4 marks)

A base reacts with an acid in a neutralisation reaction, producing a salt and water. The mixture warms up as the reaction is exothermic.

When a base reacts with an ammonium salt (e.g. NH4Cl) the salt is heated and the base deprotonates the ammonium ion, giving ammonia gas. The ammonia has a pungent, choking smell and turns damp red litmus paper blue.

Examiner tips

  • Use the word ‘neutralisation’ for the acid–base reaction. Include the exothermic temperature rise. State that heating is required for the base–ammonium salt reaction. Mention the smell of ammonia and the colour change of red litmus paper.

Common mistakes

  • Writing the reaction as a simple ion exchange without mentioning salt and water. Forgetting to note that the acid–base reaction is exothermic. Omitting the need to heat the ammonium salt or the smell of ammonia.

Mark scheme (4 marks)

  1. A base reacts with an acid in a neutralisation reaction
  2. Observation for acid–base reaction: the mixture warms up / temperature rises (exothermic)
  3. A base reacts with an ammonium salt (when heated) to produce ammonia gas
  4. Observation for base–ammonium salt reaction: ammonia has a pungent/choking smell and turns damp red litmus paper blue

Key terms in this question

base · ammonium salt

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