# A student tests two solutions, both with a pH of 3, using universal indicator. One solution contains hydrochloric acid and the other contains ethanoic acid. The student then adds a small piece of magnesium ribbon to each solution. Describe and explain what the student would observe when the magnesium is added, and explain why both solutions have the same pH despite being different types of acid.

> Edexcel GCSE Chemistry (1CH0) — 3.1 Acids · Describe and explain · 4 marks

> Hydrochloric acid is a strong acid. Ethanoic acid is a weak acid. Both solutions have been prepared so that they have a pH of 3.

## Mark scheme (4 marks)

1. The magnesium reacts faster / more vigorously in the hydrochloric acid than in the ethanoic acid (more bubbles / faster fizzing seen in hydrochloric acid)
2. Hydrochloric acid is completely ionised in aqueous solution, so it has a higher concentration of H⁺ ions available than ethanoic acid at the same pH
3. Ethanoic acid is only partially ionised in aqueous solution, so only a small number of H⁺ ions are released at any one time
4. Both solutions have the same pH because pH measures the concentration of H⁺ ions in solution, and both solutions have been prepared to have the same H⁺ ion concentration / the same number of free H⁺ ions in solution at that moment

## Related

- [Revision notes for Edexcel GCSE Chemistry (1CH0)](https://www.gradenine.co.uk/learn)
- [How to answer "Describe and 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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