Which of the following substances is not a salt?

Analyze the question: We need to find the substance that is not a salt.

Concept: A salt is a compound formed when the replaceable hydrogen ion(s) of an acid are wholly or partly replaced by metal ions or the ammonium ion. Salts are typically made from an acid plus a base, metal, or carbonate. An oxide is a compound of oxygen with one other element, so it is not a salt.

Going through the options:

• A. Aluminium oxide (Al₂O₃) is a binary compound of aluminium and oxygen. It is not formed from an acid by replacing its hydrogen, so it is an oxide, not a salt. (It is amphoteric, meaning it reacts with both acids and bases to form salts, but it is not itself a salt.)
• B. Sodium hydrogentrioxosulphate (IV) is NaHSO₃, an acid salt, where only one of the two hydrogens of H₂SO₃ is replaced by sodium. (The question writes "(V)", which is a typo, since sulphur is in the +4 oxidation state in the trioxosulphate ion. Regardless, the name is that of a salt.)
• C. Sodium trioxocarbonate (IV) is Na₂CO₃, a normal salt of carbonic acid. (The "(V)" here is also a typo. Carbon is +4.)
• D. Zinc chloride (ZnCl₂) is a normal salt of hydrochloric acid.

Answer: Options B, C and D are all salts. Aluminium oxide is an oxide, so it is the one that is not a salt.

Tip: Look at the name. If it ends in -ide and the only anion is oxide (or hydroxide), it is not a salt. Salts have names like chloride, sulphate, carbonate, or nitrate. Oxides are oxides, and hydroxides are bases. Note that chloride, sulphide and similar "-ide" names are salts, so the shortcut only applies to oxide and hydroxide.