Two soluble salts are mixed so that an insoluble salt forms as a precipitate, for example Pb(NO3)2(aq) + 2KI(aq) → PbI2(s) + 2KNO3(aq); the ionic equation is Pb2+(aq) + 2I−(aq) → PbI2(s).
An insoluble salt cannot be crystallised out of a single solution, so it is made by precipitation, also called double decomposition. Two soluble salts are mixed; their ions swap partners, and the combination that is insoluble drops out as a solid precipitate. This method, and the ionic equations behind it, are examined every year in 4541.
The balanced equation
The general word equation is: soluble salt A + soluble salt B → insoluble salt (precipitate) + soluble salt. With correct formulae and state symbols, common examples are:
Pb(NO3)2(aq) + 2KI(aq) → PbI2(s) + 2KNO3(aq)
BaCl2(aq) + Na2SO4(aq) → BaSO4(s) + 2NaCl(aq)
AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq)
The precipitate is the star of the ionic equation, which keeps only the ions that actually combine:
Pb2+(aq) + 2I−(aq) → PbI2(s)
Ba2+(aq) + SO42−(aq) → BaSO4(s)
Ag+(aq) + Cl−(aq) → AgCl(s)
Everything else, the nitrate, potassium, sodium or chloride spectator ions, stays in solution and is cancelled. Writing the correct charges and balancing them is the whole skill.
Conditions required
No heat and no catalyst are needed; precipitation happens instantly on mixing two aqueous solutions at room temperature. After mixing, the precipitate is filtered off, washed with distilled water to remove soluble impurities, and dried between filter papers or in an oven.
What you observe
The clearest sign is a precipitate forming the moment the solutions meet, the mixture turns cloudy and a solid settles. The colour identifies the salt: lead(II) iodide is a bright yellow precipitate, barium sulfate and silver chloride are white precipitates, lead(II) chloride is white, and copper(II) or iron(III) salts give coloured precipitates in related tests. There is no gas and usually no temperature change worth noting.
Where it appears in the SPM exam
In 4541/2 precipitation is the method asked for whenever an insoluble salt must be prepared, and it is the setting for writing ionic equations. It is also central to qualitative analysis: adding silver nitrate to test for halide ions, or barium chloride to test for sulfate ions, are precipitation reactions with signature colours. In 4541/1 solubility rules and precipitate colours are tested directly, and in 4541/3 you may prepare and identify a precipitate.
How we teach it
We start from the solubility rules so you can predict which product is insoluble, then practise stripping the full equation down to its ionic equation with correct charges. The most common mistakes are forgetting the state symbol (s) on the precipitate, using the wrong charge on lead or barium, and not balancing the spectator salt. One tutor tip we repeat is to name the precipitate and its colour in the same breath, because the exam usually wants both.
Quick summary
Remember the shape of the reaction: two clear solutions in, one coloured or white solid out, with the ionic equation showing only the ions that join. Learn a few standard pairs, lead(II) iodide (yellow), barium sulfate (white), silver chloride (white), and the solubility rules that predict them, and you can answer preparation, ionic-equation and qualitative-analysis questions from a single, secure idea.
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