# A student dissolves ammonia gas in water to produce an alkaline solution. Explain why ammonia solution is described as a weak base, and describe how the pH of ammonia solution compares to that of sodium hydroxide solution of the same concentration.

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

> Ammonia (NH₃) dissolves readily in water and acts as a base. Sodium hydroxide (NaOH) is also a base.

## Mark scheme (5 marks)

1. Ammonia is a weak base because it only partially dissociates (ionises) in water
2. So the equilibrium in water lies to the left (reactants favoured)
3. This means the concentration of hydroxide ions (OH⁻) in ammonia solution is low / lower than expected
4. Sodium hydroxide is a strong base because it fully / completely dissociates in water
5. Ammonia solution has a lower pH than sodium hydroxide solution of the same concentration (because it produces fewer OH⁻ ions)

## Key terms

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

## 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/a-student-dissolves-ammonia-gas-in-70fe51af) · Published by Druglandscape Ltd.