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 · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (5 marks)

Magnesium oxide is an ionic compound, consisting of Mg²⁺ and O²⁻ ions. When it is added to water it dissociates to give O²⁻, which reacts with H₂O to form OH⁻, producing a basic solution (pH > 7). Phosphorus(V) oxide, on the other hand, is covalently bonded; its P–O bonds are polar covalent. In water it hydrolyses to give H₃PO₄, releasing H⁺ ions and giving an acidic solution (pH < 7). Thus the different behaviour arises from the type of bonding: ionic MgO gives a basic hydroxide, while covalent P₂O₅ gives an acidic phosphoric acid. This illustrates the shift from basic to acidic oxides across Period 3 as the elements change from metallic to non‑metallic character.

Examiner tips

  • Mention ionic vs covalent bonding first; state the products (Mg(OH)₂ vs H₃PO₄); link to pH change; keep answer concise and use correct terminology.
  • Use the word ‘hydrolyses’ for P₂O₅ and ‘dissociates’ for MgO to show understanding of reactions.

Common mistakes

  • Confusing MgO with Mg(OH)₂ as a separate compound; writing that MgO is acidic.
  • Saying P₂O₅ is ionic or ignoring its covalent nature; not mentioning the acidic pH.

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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