Insoluble Salt – IGCSE Chemistry Definition

An insoluble salt is a salt that does not dissolve in water (or dissolves only to a negligible extent). At IGCSE level, common insoluble salts include silver chloride (AgCl), lead(II) chloride (PbCl₂), barium sulfate (BaSO₄), lead(II) sulfate (PbSO₄), calcium sulfate (CaSO₄), and most carbonates (except those of sodium, potassium, and ammonium).

Preparation by precipitation

Insoluble salts cannot be made by the methods used for soluble salts (excess solid or titration) because the product would not dissolve in the acid. Instead, they are prepared by a precipitation reaction: two soluble solutions are mixed so that the required insoluble salt forms as a solid precipitate.

For example, to make lead(II) iodide, mix lead(II) nitrate solution with potassium iodide solution. A bright yellow precipitate of lead(II) iodide forms immediately.

Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) + 2KNO₃(aq)

The ionic equation: Pb²⁺(aq) + 2I⁻(aq) → PbI₂(s)

The precipitate is then collected by filtration, washed with distilled water to remove any soluble impurities, and dried.

Choosing the right solutions

The two starting solutions must each contain one of the required ions and must both be soluble. For silver chloride, use silver nitrate (provides Ag⁺) and sodium chloride (provides Cl⁻). The other ions (Na⁺ and NO₃⁻) remain in solution and are washed away.

Exam context

Paper 4 questions may ask candidates to describe the preparation of a named insoluble salt. Marks are awarded for naming appropriate soluble starting solutions, describing the mixing, filtration, washing, and drying steps.

Worked exam question

Describe how to prepare a sample of barium sulfate from barium chloride and sodium sulfate solutions. (3 marks)

Mix barium chloride solution with sodium sulfate solution. A white precipitate of barium sulfate forms [1]. Filter the mixture to collect the precipitate on filter paper [1]. Wash the precipitate with distilled water and dry in a warm oven or between filter papers [1].

Common mistakes

Candidates sometimes choose insoluble starting materials, which defeats the purpose. Both starting solutions must be soluble so the ions are free to meet and precipitate. Another error is forgetting to wash the precipitate with distilled water. Without washing, soluble impurities (such as NaCl or KNO₃) remain on the crystals.