Describe how you would use universal indicator solution to distinguish between a dilute acid, a neutral solution, and an alkali. Include the colours you would expect to observe.

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)

Add a few drops of universal indicator solution to each sample.
1. Dilute acid – the solution turns red/orange (pH < 7).
2. Neutral solution – the solution turns green (pH = 7).
3. Alkali – the solution turns blue/purple/violet (pH > 7).

Examiner tips

  • Use the command word ‘describe’ – list the steps and the expected colours. Mention that the indicator is added to each sample separately. Show the colour change for each pH range as per the mark scheme.

Common mistakes

  • Confusing the colours for acid and alkali. Failing to state that the indicator is added to each sample. Using the wrong colour for a neutral solution (e.g. blue instead of green).

Mark scheme (4 marks)

  1. Add universal indicator solution to each sample / to the solution being tested
  2. Dilute acid turns universal indicator red / orange (pH below 7)
  3. Neutral solution turns universal indicator green (pH 7)
  4. Alkali turns universal indicator blue / purple / violet (pH above 7)

Key terms in this question

universal indicator · acid · neutral · alkali

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