Salt – IGCSE Chemistry Definition

Salt preparation is one of the most heavily examined practical topics in IGCSE Chemistry. Paper 4 frequently asks candidates to describe how to prepare a named salt and choose the correct method. Knowing which method to use — and why — is worth 4–6 marks.

The 0620 definition

A salt is a compound formed when the hydrogen ion(s) of an acid are replaced by a metal ion or an ammonium ion (NH₄⁺).

For example, in HCl → NaCl, the H⁺ has been replaced by Na⁺.

Naming salts

Acid used	Anion produced	Example salt
Hydrochloric acid, HCl	Chloride, Cl⁻	Magnesium chloride, MgCl₂
Sulfuric acid, H₂SO₄	Sulfate, SO₄²⁻	Copper(II) sulfate, CuSO₄
Nitric acid, HNO₃	Nitrate, NO₃⁻	Zinc nitrate, Zn(NO₃)₂

Three methods of preparing salts

Method 1: Acid + excess insoluble substance (for soluble salts)

Used when the base, metal oxide, or carbonate is insoluble. Add excess solid to warm acid, filter off the unreacted solid, then crystallise.

Example: CuO + H₂SO₄ → CuSO₄ + H₂O

Method 2: Titration (for soluble salts from two solutions)

Used when both reactants are soluble (acid + alkali). Use a titration with an indicator to find the exact volumes, then repeat without indicator and crystallise.

Example: NaOH + HCl → NaCl + H₂O

Method 3: Precipitation (for insoluble salts)

Mix two soluble salt solutions. The insoluble salt forms as a precipitate, which is filtered, washed with distilled water, and dried.

Example: Pb(NO₃)₂(aq) + 2NaCl(aq) → PbCl₂(s) + 2NaNO₃(aq)

Choosing the method

The question to ask: is the salt you want soluble or insoluble?

• Soluble salt from an insoluble reactant → Method 1
• Soluble salt from two soluble reactants → Method 2
• Insoluble salt → Method 3

Worked exam question

Describe how to prepare a pure, dry sample of lead(II) iodide from lead(II) nitrate solution and potassium iodide solution. [4]

Mark scheme

Mix lead(II) nitrate solution with potassium iodide solution [1]; a yellow precipitate of lead(II) iodide forms [1]; filter the mixture to collect the precipitate [1]; wash the precipitate with distilled water and dry between filter papers / in a warm oven [1]

Common exam mistakes

• Choosing Method 1 (excess solid + filter) when both reactants are solutions — you cannot filter out excess liquid, so a titration is needed.
• Forgetting to wash the precipitate in Method 3. Without washing, soluble impurities (e.g. NaNO₃) remain.
• Writing the formula of the sulfate salt wrong — sulfate is SO₄²⁻ not SO₃²⁻ (that is sulfite).