Period 3 Oxides and Chlorides

Building on the reactions from the previous lesson, we now examine the behaviour of Period 3 oxides and chlorides in more detail — specifically their reactions with water and their acid-base properties. These reactions are a staple of Edexcel A-Level examination questions.

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Reactions of Period 3 Oxides with Water

Sodium Oxide (Na₂O) — Strongly Basic

Na₂O dissolves readily in water to form a strongly alkaline solution:

Na₂O(s) + H₂O(l) → 2NaOH(aq)

The oxide ion (O²⁻) is a strong base that abstracts a proton from water. NaOH is a strong alkali — it fully dissociates, giving pH ≈ 13–14.

Magnesium Oxide (MgO) — Basic

MgO reacts slowly with water (it is sparingly soluble):

MgO(s) + H₂O(l) → Mg(OH)₂(aq)

Mg(OH)₂ is only slightly soluble, so the resulting solution is weakly alkaline (pH ≈ 9–10). Despite its limited solubility, MgO is classified as a basic oxide.

Aluminium Oxide (Al₂O₃) — Amphoteric

Al₂O₃ does not dissolve in water. It is insoluble because the strong ionic/covalent bonding in the lattice requires too much energy to break.

However, Al₂O₃ is amphoteric — it reacts with both acids and bases:

With acid: Al₂O₃(s) + 6HCl(aq) → 2AlCl₃(aq) + 3H₂O(l)