# Explain why the pH of an aqueous solution formed when a Period 3 oxide dissolves in water changes across the period from sodium to sulfur, referring to the oxides of sodium, magnesium, and sulfur in your answer.

> AQA A-Level Chemistry (7405) — 3.2.4 Properties of Period 3 elements and their oxides (A-Level only) · Explain · 5 marks

## Mark scheme (5 marks)

1. Sodium oxide (or magnesium oxide) is a basic/alkaline oxide that reacts with water to produce OH⁻ ions (or an alkaline solution with pH above 7)
2. Sodium and magnesium oxides are ionic compounds / contain metal–oxygen ionic bonds, so the oxide ion (O²⁻) reacts with water to form OH⁻
3. Across the period the bonding in the oxides changes from ionic to covalent (as the elements become non-metals)
4. Sulfur oxide (SO₂ / SO₃) is an acidic oxide that reacts with water to produce H⁺ ions / an acidic solution with pH below 7
5. Therefore the pH of the solution decreases (from above 7 to below 7) as you move from sodium to sulfur across Period 3

## Key terms

- [Period 3](https://www.gradenine.co.uk/glossary/period-3)
- [pH](https://www.gradenine.co.uk/glossary/ph)

## Related

- [Revision notes for AQA A-Level Chemistry (7405)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-the-ph-of-an-8ae3f25e) · Published by Druglandscape Ltd.