Explain what happens when hydrochloric acid is added to a solution of sodium hydroxide, in terms of the ions present and the formation of a salt.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Hydrochloric acid and sodium hydroxide solution are mixed together in equal volumes with equal concentrations.
Model answer (5 marks)
Hydrochloric acid dissociates to give H⁺ and Cl⁻.
Sodium hydroxide dissociates to give Na⁺ and OH⁻.
The H⁺ and OH⁻ ions combine to form water (H⁺ + OH⁻ → H₂O) – a neutralisation reaction.
The Na⁺ and Cl⁻ ions remain in solution and form the salt sodium chloride (NaCl).
With equal concentrations and volumes the numbers of H⁺ and OH⁻ are equal, so the solution is neutral (pH 7).
Sodium hydroxide dissociates to give Na⁺ and OH⁻.
The H⁺ and OH⁻ ions combine to form water (H⁺ + OH⁻ → H₂O) – a neutralisation reaction.
The Na⁺ and Cl⁻ ions remain in solution and form the salt sodium chloride (NaCl).
With equal concentrations and volumes the numbers of H⁺ and OH⁻ are equal, so the solution is neutral (pH 7).
Examiner tips
- Use the word ‘dissociates’ for both acids and bases. Show the ion‑pair reaction H⁺ + OH⁻ → H₂O. Mention the remaining ions form NaCl. State the final pH is 7 because of stoichiometric equivalence.
Common mistakes
- Writing ‘reacts’ instead of ‘dissociates’ for the initial step. Omitting the formation of NaCl. Failing to note that the final solution is neutral (pH 7).
Mark scheme (5 marks)
- Hydrochloric acid dissociates/ionises to produce hydrogen ions (H⁺) and chloride ions (Cl⁻)
- Sodium hydroxide dissociates/ionises to produce hydroxide ions (OH⁻) and sodium ions (Na⁺)
- The hydrogen ions and hydroxide ions react together to form water (H⁺ + OH⁻ → H₂O) — this is the neutralisation reaction
- The sodium ions (Na⁺) and chloride ions (Cl⁻) remain in solution and form the salt sodium chloride (NaCl)
- Because equal concentrations and equal volumes are used, the resulting solution is neutral (pH 7)
Key terms in this question
Related
- All AQA A-Level Chemistry (7405) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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