# 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.

> OCR A-Level Chemistry A (H432) — 2.3 Acid-base and redox reactions · Explain · 5 marks

> 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)

## Mark scheme (5 marks)

1. The reaction is a neutralisation reaction
2. An acid reacts with a base (alkali) to form a salt and water
3. The pH decreases / falls as hydrochloric acid is added
4. At the point the reaction is complete / at the equivalence point, the pH is 7 (neutral)
5. After the reaction is complete, adding excess acid causes the pH to fall below 7 / the solution becomes acidic

## Key terms

- [pH](https://www.gradenine.co.uk/glossary/ph)
- [acid](https://www.gradenine.co.uk/glossary/acid)

## Related

- [Revision notes for OCR A-Level Chemistry A (H432)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-student-adds-dilute-hydrochloric-acid-bc7853b4) · Published by Druglandscape Ltd.