Which of the following substances is not a salt?

Correct answer: Aluminium oxide

Explanation: A salt is formed when the hydrogen ion(s) in an acid are wholly or partially replaced by a metal or ammonium ion. Aluminium oxide (Al₂O₃) is an oxide, not a salt. The other options are typical salts:

• Aluminium oxide — Not a salt; it is a metal oxide.
• Sodium hydrogen trioxosulphate(V) (NaHSO₄) — An acid salt.
• Sodium trioxocarbonate(V) (Na₂CO₃) — A normal salt (sodium carbonate).
• Zinc chloride (ZnCl₂) — A typical ionic salt.