Soluble Salt – IGCSE Chemistry Definition

A soluble salt is a salt that dissolves in water to form an aqueous solution. At IGCSE level, candidates must know the solubility rules to predict whether a salt is soluble or insoluble, and they must be able to describe methods for preparing soluble salts.

Solubility rules

All sodium, potassium, and ammonium salts are soluble. All nitrates are soluble. Most chlorides are soluble (except silver chloride and lead(II) chloride). Most sulfates are soluble (except barium sulfate, calcium sulfate, and lead(II) sulfate). Most carbonates are insoluble (except sodium, potassium, and ammonium carbonates).

Preparation methods

There are two main methods for preparing soluble salts, depending on whether the base or metal used is soluble or insoluble.

Method 1 (excess solid method): Add excess insoluble base (e.g. copper oxide), metal oxide, carbonate, or reactive metal to a warm acid. The excess solid ensures all the acid reacts. Filter to remove the unreacted excess solid. Evaporate some water from the filtrate, then allow the solution to cool and crystallise. Filter off the crystals and dry them.

Method 2 (titration method): Used when both the acid and the base are soluble (e.g. making sodium chloride from NaOH and HCl). Find the exact volumes needed by titrating with an indicator. Repeat the titration without the indicator using the exact volumes. Evaporate the solution to obtain the salt.

Exam context

Paper 4 questions frequently ask candidates to describe how to prepare a named soluble salt, awarding marks for each correct procedural step. The choice of method depends on whether the starting base is soluble or insoluble.

Worked exam question

Describe how you would prepare a pure, dry sample of zinc sulfate from zinc oxide and dilute sulfuric acid. (4 marks)

Add excess zinc oxide to warm dilute sulfuric acid and stir [1]. Filter the mixture to remove excess zinc oxide [1]. Heat the filtrate gently to evaporate some water [1]. Allow to cool to crystallise, then filter and dry the crystals [1].

Common mistakes

Candidates sometimes suggest titration when one reactant is an insoluble solid. Titration is only needed when both reactants are soluble (e.g. NaOH + HCl). If an insoluble base is available, the excess solid method is simpler and avoids indicator contamination. Another error is evaporating all the water at once, which gives a powder, not crystals.