# Explain why hydrochloric acid is described as a strong acid, and why ethanoic acid is described as a weak acid. In your answer, refer to the degree of ionisation of each acid in water.

> WJEC A-Level Chemistry (Wales) — 3.9 Acid-base equilibria · Explain · 5 marks

## Mark scheme (5 marks)

1. Hydrochloric acid completely / fully ionises in water
2. All / nearly all HCl molecules produce H⁺ ions (and Cl⁻ ions) in solution
3. Ethanoic acid only partially / incompletely ionises in water
4. Only a small proportion of ethanoic acid molecules release H⁺ ions in solution / an equilibrium is established with mostly un-ionised molecules present
5. Therefore hydrochloric acid has a higher concentration of H⁺ ions than ethanoic acid at the same concentration, so has a lower pH

## Key terms

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

## Related

- [Revision notes for WJEC A-Level Chemistry (Wales)](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/explain-why-hydrochloric-acid-is-described-e04e5cd8) · Published by Druglandscape Ltd.