# Explain why magnesium oxide and phosphorus(V) oxide show different behaviour when added to water, referring to the type of bonding and structure in each oxide.

> 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. Magnesium oxide has ionic bonding (between Mg²⁺ and O²⁻ ions)
2. Magnesium oxide reacts with water to form magnesium hydroxide / OH⁻ ions, producing an alkaline solution (pH > 7)
3. Phosphorus(V) oxide has covalent bonding (between non-metal atoms)
4. Phosphorus(V) oxide reacts with water to form phosphoric acid / H⁺ ions, producing an acidic solution (pH < 7)
5. The change from alkaline to acidic behaviour across Period 3 oxides reflects the change from metallic (ionic) to non-metallic (covalent) character / the type of oxide changes from basic to acidic across the period

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