# A student dissolves citric acid in water and measures the pH. She then dissolves the same amount of hydrochloric acid in the same volume of water and measures the pH. Both solutions have the same concentration. Explain why the two solutions have different pH values, including the terms weak acid and strong acid in your answer.

> OCR A-Level Chemistry B: Salters (H433) — 3.2 Physical chemistry · Explain · 5 marks

> Citric acid is found in lemons. Hydrochloric acid is used in industry. Both are acids, but their behaviour in water is different.

## Mark scheme (5 marks)

1. Hydrochloric acid is a strong acid
2. Hydrochloric acid is completely ionised in aqueous solution, releasing (almost) all of its H⁺ ions
3. Citric acid is a weak acid
4. Citric acid is only partially ionised in aqueous solution, releasing only a small number of H⁺ ions
5. The hydrochloric acid solution has a higher concentration of H⁺ ions, so it has a lower pH than the citric acid solution

## Key terms

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

## 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)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/a-student-dissolves-citric-acid-in-6546f9ed) · Published by Druglandscape Ltd.