# 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

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

## 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

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

## Related

- [Revision notes for OCR A-Level Chemistry B: Salters (H433)](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-tests-an-unknown-solution-f1dad210) · Published by Druglandscape Ltd.