Explain why aluminium oxide does not dissolve in water, yet reacts with both dilute hydrochloric acid and dilute sodium hydroxide solution.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Aluminium oxide is amphoteric.
Its structure is a giant ionic lattice with strong electrostatic forces between Al^3+ and O^2− ions, so it does not dissolve in water.
In dilute HCl it acts as a base: Al_2O_3 + 6HCl → 2AlCl_3 + 3H_2O.
In dilute NaOH it acts as an acid: Al_2O_3 + 2NaOH + 3H_2O → 2Na[Al(OH)_4].
The amphoteric character arises because Al is in the middle of Period 3, between the metals that form basic oxides and the non‑metals that form acidic oxides.
Its structure is a giant ionic lattice with strong electrostatic forces between Al^3+ and O^2− ions, so it does not dissolve in water.
In dilute HCl it acts as a base: Al_2O_3 + 6HCl → 2AlCl_3 + 3H_2O.
In dilute NaOH it acts as an acid: Al_2O_3 + 2NaOH + 3H_2O → 2Na[Al(OH)_4].
The amphoteric character arises because Al is in the middle of Period 3, between the metals that form basic oxides and the non‑metals that form acidic oxides.
Examiner tips
- State that Al_2O_3 is amphoteric and give the two reaction equations; this shows understanding of both acid and base behaviour.
- Explain the lattice structure to justify insolubility in water; examiners reward clear reasoning.
- Mention Al’s position in Period 3 to link structure to reactivity.
- Use correct chemical formulae and stoichiometry.
Mark scheme (5 marks)
- Aluminium oxide is amphoteric
- Aluminium oxide has a giant ionic/lattice structure with strong electrostatic forces between ions, so it does not dissolve in water
- Aluminium oxide reacts with dilute hydrochloric acid, acting as a base
- Aluminium oxide reacts with dilute sodium hydroxide solution, acting as an acid
- The amphoteric nature arises because aluminium is in the middle of Period 3, between the metals (which form basic oxides) and non-metals (which form acidic oxides)
Related
- All AQA A-Level Chemistry (7405) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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