Classify $mathrm{Al}_2 mathrm{O}_3$ and $mathrm{ZnO}$ as amphoteric oxides

Acids, Bases and Salts – Oxides (IGCSE 0620)

Learning Objective

Identify Al₂O₃ and ZnO as the two amphoteric oxides listed in the core Cambridge IGCSE Chemistry 0620 syllabus and write their reactions with both acids and bases.

Key Properties of Al₂O₃ and ZnO

• White, solid oxides (s).
• Insoluble in cold water – they only “dissolve” when they react with an acid or a strong base.
• React with strong acids → soluble salt + water (the oxide behaves as a base).
• React with strong bases → soluble hydroxocomplex (the oxide behaves as an acid).

Acid‑Base Notation Reminder

For any amphoteric oxide:

• Oxide + H⁺ → salt + H₂O (behaviour of a base)
• Oxide + OH⁻ → hydroxocomplex (behaviour of an acid)

Reactions with Acids (Oxide acts as a base)

Reactions with Bases (Oxide acts as an acid)

Comparison of the Two Core Amphoteric Oxides

Why Are Al₂O₃ and ZnO Amphoteric?

• Polarising power: Al³⁺ (small, high charge) and Zn²⁺ (small, relatively high charge) strongly polarise the O²⁻ ion, weakening the O–H bond formation and allowing the oxide to either accept H⁺ (behave as a base) or donate O²⁻ to OH⁻ (behave as an acid).
• Periodic‑table position: Both elements lie near the metal–non‑metal boundary (Al in Group 13, Zn in Group 12/transition‑metal region). Oxides of such elements are typically neither strongly acidic nor strongly basic, giving them amphoteric character.

Solubility Note

Both the simple salts (AlCl₃, ZnCl₂) and the hydroxocomplexes (Na[Al(OH)₄]⁻, Na₂[Zn(OH)₄]²⁻) are soluble in water. This explains the observable fact that Al₂O₃ and ZnO, which are insoluble in cold water, “dissolve” when treated with a strong acid or a strong base.

Other Amphoteric Oxides (Supplementary material)

Link to Core Syllabus 7.1 – Acids & Bases

• All aqueous acids contain hydrogen ions (H⁺). When an amphoteric oxide reacts with an acid it **accepts H⁺**, forming a salt and water – the oxide is acting as a **base**.
• All aqueous bases contain hydroxide ions (OH⁻). When an amphoteric oxide reacts with a base it **donates O²⁻** (or forms a complex) and incorporates OH⁻ into a soluble anion – the oxide is acting as an **acid**.

Exam Tips

• Look for key words: “reacts with HCl”, “dissolves in NaOH”, “forms a complex ion”.
• Write **balanced** equations and always include state symbols (s, aq, l).
• Identify the product:
  • Simple salt + water → oxide behaved as a **base**.
  • Soluble hydroxocomplex (e.g., [Al(OH)₄]⁻) → oxide behaved as an **acid**.
• Remember: the oxides are **insoluble in cold water** but become soluble when they react with either a strong acid or a strong base – this is often the clue the exam question is testing.