A student tests an unknown solution using universal indicator and finds it has a pH of 2. The student then adds excess sodium hydroxide solution to the unknown solution. Explain what happens to the pH of the solution as sodium hydroxide is added, and describe the type of reaction that is occurring.

OCR A-Level Chemistry B: Salters (H433) — 2.3 Acid-base and redox reactions · Explain · 5 marks · View as Markdown

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

An unknown solution has a pH of 2. Sodium hydroxide solution is added to it in excess.

Model answer (5 marks)

The solution is a strong acid, as indicated by its pH of 2.

Adding NaOH, a strong base, causes the pH to rise.

At the equivalence point the acid and base are present in stoichiometric amounts and the pH is 7.

With further NaOH the pH rises above 7, making the solution alkaline.

The reaction is a neutralisation (acid–base) reaction between the acid and the alkali.

Examiner tips

  • Use the exact terms: strong acid, strong base, neutralisation, equivalence point, pH rise.
  • Show the pH trend: low → 7 → >7.
  • Mention the stoichiometric balance at equivalence.
  • Keep the answer concise and to the point.

Common mistakes

  • Saying the pH stays at 2 after adding NaOH.
  • Confusing the type of reaction (e.g., calling it a redox).
  • Not recognising the equivalence point where pH = 7.

Mark scheme (5 marks)

  1. A pH of 2 shows the unknown solution is a (strong) acid
  2. As sodium hydroxide is added, the pH increases / rises
  3. At the equivalence/neutralisation point, the pH reaches 7 (neutral)
  4. With excess sodium hydroxide, the pH rises above 7 / the solution becomes alkaline
  5. The reaction is a neutralisation reaction (between an acid and an alkali/base)

Key terms in this question

pH · excess

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