An insoluble salt can be prepared by

Analyze the question: It asks for the method used to prepare an insoluble salt.

Concept: Salts are prepared by different methods depending on their solubility:

• Soluble salts are made by acid + metal, acid + base/alkali, or acid + carbonate. The salt stays dissolved and is recovered by evaporation and crystallisation.
• Insoluble salts are made by precipitation, also called double decomposition. Two soluble salt solutions are mixed, the ions swap partners, and one of the new combinations is insoluble and precipitates out. It is then filtered, washed and dried.

General form: AB(aq) + CD(aq) → AD(s) + CB(aq)

Check the options:

• A. Trioxocarbonate(V) with an acid: This gives a salt, water and CO₂. It is a method for soluble salts. Also, if the carbonate is insoluble, the reaction usually dissolves it. ✗
• B. Double decomposition: This is the precipitation method for insoluble salts. ✓
• C. Dilute acid on an insoluble base: This forms a soluble salt and water (e.g. CuO + H₂SO₄ → CuSO₄ + H₂O). ✗
• D. Metals with an acid: This gives a soluble salt and hydrogen gas. It is also unsuitable for many metals, and an insoluble product would coat the metal and stop the reaction. ✗

Example (Nigerian exam favourite): Pb(NO₃)₂(aq) + 2NaCl(aq) → PbCl₂(s) + 2NaNO₃(aq) Another: BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq)

**Answer: B. Double decomposition.

Tip: In exams, link "insoluble salt" with "precipitation / double decomposition / mixing two solutions." Link "soluble salt" with "evaporation and crystallisation."