A student adds dilute hydrochloric acid to sodium hydroxide solution. Explain what type of reaction is taking place and describe what happens to the pH of the sodium hydroxide solution as hydrochloric acid is gradually added until the reaction is complete.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Sodium hydroxide solution has a pH of 14. A student adds dilute hydrochloric acid drop by drop, stirring continuously, until the reaction is complete. The equation for the reaction is: NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)
Model answer (5 marks)
The reaction is a neutralisation reaction: an acid reacts with a base (alkali) to form a salt and water.
As dilute hydrochloric acid is added, the pH of the sodium hydroxide solution falls.
When the reaction is complete – the equivalence point – the pH is 7 (neutral).
If more acid is added beyond the equivalence point, the solution becomes acidic and the pH falls below 7.
As dilute hydrochloric acid is added, the pH of the sodium hydroxide solution falls.
When the reaction is complete – the equivalence point – the pH is 7 (neutral).
If more acid is added beyond the equivalence point, the solution becomes acidic and the pH falls below 7.
Examiner tips
- State it is a neutralisation reaction first. Show the pH trend: decreases → 7 at equivalence → below 7 if excess acid.
- Use the exact wording from the mark scheme (neutralisation, salt, water, pH 7).
Common mistakes
- Confusing the reaction type (e.g., calling it a precipitation). Forgetting to mention the pH reaches 7 at the equivalence point. Not recognising that excess acid makes the solution acidic.
Mark scheme (5 marks)
- The reaction is a neutralisation reaction
- An acid reacts with a base (alkali) to form a salt and water
- The pH decreases / falls as hydrochloric acid is added
- At the point the reaction is complete / at the equivalence point, the pH is 7 (neutral)
- After the reaction is complete, adding excess acid causes the pH to fall below 7 / the solution becomes acidic
Key terms in this question
Related
- All OCR A-Level Chemistry A (H432) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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