Precipitate – IGCSE Chemistry Definition and Examples

Precipitate questions appear on Papers 2 and 4 in two main contexts: identifying ions by the precipitate colour they produce, and writing ionic equations for precipitation reactions. Candidates who learn the handful of solubility rules and the colour chart find these questions predictable.

The 0620 definition

A precipitate is an insoluble solid formed when two aqueous solutions are mixed. The ions in the two solutions combine to form a compound that cannot dissolve, so it appears as a solid suspended in the liquid. The mixture looks cloudy or turbid, and the solid can be collected by filtration.

Example: mixing silver nitrate solution with sodium chloride solution produces a white precipitate of silver chloride.

AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq)

The ionic equation removes the spectator ions (Na⁺ and NO₃⁻) that remain dissolved:

Ag⁺(aq) + Cl⁻(aq) → AgCl(s)

Solubility rules you need

To predict precipitates, memorise these rules from the syllabus:

Rule	Detail
All sodium, potassium and ammonium salts	Soluble — never form precipitates
All nitrates	Soluble
Most chlorides	Soluble, except lead(II) chloride and silver chloride
Most sulfates	Soluble, except barium sulfate, calcium sulfate and lead(II) sulfate
Most carbonates	Insoluble, except sodium, potassium and ammonium carbonates
Most hydroxides	Insoluble, except sodium, potassium and calcium hydroxides

When two solutions are mixed, check whether the possible products include any insoluble compound. If yes, that compound is the precipitate.

Precipitate colours tested in 0620

Precipitate	Colour	When it forms
Silver chloride, AgCl	White	Adding AgNO₃ to a chloride solution
Silver bromide, AgBr	Cream	Adding AgNO₃ to a bromide solution
Silver iodide, AgI	Yellow	Adding AgNO₃ to an iodide solution
Barium sulfate, BaSO₄	White	Adding BaCl₂ to a sulfate solution
Lead(II) iodide, PbI₂	Bright yellow	Adding Pb(NO₃)₂ to a potassium iodide solution
Copper(II) hydroxide, Cu(OH)₂	Blue	Adding NaOH to a Cu²⁺ solution
Iron(II) hydroxide, Fe(OH)₂	Green	Adding NaOH to an Fe²⁺ solution
Iron(III) hydroxide, Fe(OH)₃	Red-brown	Adding NaOH to an Fe³⁺ solution

These colours are central to the qualitative analysis tests on Paper 6 and Paper 4.

Preparing an insoluble salt by precipitation

The standard method for making an insoluble salt:

1. Mix two solutions that contain the required ions (e.g. lead(II) nitrate and potassium iodide to make lead(II) iodide).
2. Filter the mixture to collect the precipitate as the residue.
3. Wash the residue with distilled water to remove soluble impurities.
4. Dry the precipitate between filter papers or in a warm oven.

This method works because the product is insoluble — it cannot be made by the acid-plus-base route used for soluble salts.

Common exam mistakes

1. Confusing precipitate with residue. A precipitate specifically forms during a reaction between solutions; a residue is any solid left after filtration, which may or may not have been formed by precipitation.
2. Forgetting state symbols. The precipitate must be shown as (s) in the equation; writing (aq) for the product that just formed as a solid loses the mark.
3. Writing the full equation when the question asks for the ionic equation. Remove spectator ions.
4. Describing the observation as “a solid forms” without stating the colour. The colour is usually a separate marking point.