A student adds dilute sulfuric acid to a solution of sodium hydroxide. Explain what happens during this reaction, including what type of reaction is taking place and how the products are formed.

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).

Sodium hydroxide solution is a common alkali used in laboratories. When mixed with dilute sulfuric acid, a neutralisation reaction occurs, producing sodium sulfate and water.

Model answer (5 marks)

The reaction is a neutralisation (acid–base) reaction. The sulphuric acid supplies H⁺ ions, while the sodium hydroxide supplies OH⁻ ions. The H⁺ and OH⁻ ions combine to form water:
H⁺ + OH⁻ → H₂O
The remaining ions, Na⁺ and SO₄²⁻, combine to give the salt sodium sulfate (Na₂SO₄).

Examiner tips

  • Use the term ‘neutralisation’ or ‘acid–base reaction’.
  • Show the ion‑pairing step H⁺ + OH⁻ → H₂O explicitly.
  • Mention the formation of the salt Na₂SO₄ as the second product.
  • Keep the answer concise – 5 marks only.

Common mistakes

  • Writing the reaction as a single balanced equation without showing the ion‑pairing step.
  • Forgetting to identify the type of reaction (neutralisation).
  • Using the wrong salt formula (e.g., NaSO₄ instead of Na₂SO₄).

Mark scheme (5 marks)

  1. The reaction is a neutralisation (reaction) / acid-base reaction
  2. The acid provides hydrogen ions / H⁺ ions (to the reaction)
  3. The alkali / sodium hydroxide provides hydroxide ions / OH⁻ ions
  4. The hydrogen ions and hydroxide ions react together to form water (H⁺ + OH⁻ → H₂O)
  5. A salt (sodium sulfate) is also formed as a product of the reaction

Key terms in this question

acid

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