A student dissolves an unknown white solid in water. The solution is tested with Universal Indicator and a pH of 2 is recorded. The student then adds a spatula of sodium hydroxide powder to the solution and stirs. Explain what happens to the pH of the solution as the sodium hydroxide is added, and state the type of reaction occurring.

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).

Sodium hydroxide is a base. A solution with a pH of 2 is strongly acidic.

Model answer (4 marks)

The pH rises as sodium hydroxide is added.
The hydroxide ions from NaOH react with the acidic species in the solution, neutralising the acid.
With sufficient NaOH the pH will reach 7 (neutral) – if excess NaOH is added it can rise above 7.
The reaction is a neutralisation reaction.

Examiner tips

  • Show the pH change first, then explain neutralisation, mention neutral pH and possible excess.
  • Use the word ‘neutralises’ and ‘neutralisation’ as the scheme expects.
  • Include the possibility of pH >7 if excess NaOH is added.

Mark scheme (4 marks)

  1. The pH increases (as sodium hydroxide is added)
  2. Because the sodium hydroxide neutralises the acid / the alkali reacts with the acid
  3. If enough sodium hydroxide is added the pH will reach 7 (neutral) / the pH could rise above 7 if excess sodium hydroxide is added
  4. The type of reaction is neutralisation

Key terms in this question

pH

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