A student tests two solutions, X and Y, using Universal Indicator. Solution X gives a red colour and solution Y gives a purple colour. The student then adds an excess of sodium hydroxide solution to solution X and re-tests the resulting mixture with Universal Indicator, which now gives a green colour. Explain what these observations tell us about solutions X and Y, and explain what has happened chemically when sodium hydroxide was added to solution X.

WJEC GCSE Chemistry (Wales) — 2.2 Acids, bases and salts · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Solution X is acidic, giving a red colour with Universal Indicator.
Solution Y is alkaline, giving a purple colour.
Adding excess NaOH to X neutralises the acid:
acid + NaOH → salt + H₂O.
The green colour indicates the mixture is now neutral (pH≈7), showing that a salt and water have formed.

Examiner tips

  • Use the colour–pH chart to state X is acidic and Y is alkaline. Explain the neutralisation reaction and the change to green. Show the link between neutral pH and the green colour.

Common mistakes

  • Confusing the colours (e.g., saying X is basic). Failing to mention the formation of a salt and water. Not linking the green colour to a neutral pH.

Mark scheme (4 marks)

  1. Solution X is acidic / is an acid
  2. Solution Y is alkaline / is a base / has a pH above 7
  3. The sodium hydroxide neutralised the acid in solution X
  4. The green colour shows the resulting mixture is neutral / has a pH of 7, so a salt and water were formed

Key terms in this question

Universal Indicator

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