Precipitation – IGCSE Chemistry Definition
IGCSE Chemistry definition of precipitation: forming an insoluble solid by mixing two solutions. Covers solubility rules, ionic equations, and exam tips for 0620.
Published by IGCSEChemistry.com.my
Chemistry teaching team: K. S. Tan (15+ years teaching IGCSE Chemistry) and Ms Yash (10+ years teaching IGCSE Chemistry) and Ms Kartini (15+ years teaching IGCSE Chemistry).
Mapped to Cambridge IGCSE Chemistry 0620 (2026–2028). Last updated 2026-08-19.
Precipitation reactions are tested in two main contexts in IGCSE Chemistry: preparing insoluble salts (Paper 4) and identifying unknown ions through precipitate tests (Paper 6 and Paper 4). Knowing the solubility rules is essential for both.
The 0620 definition
Precipitation is the formation of an insoluble solid (a precipitate) when two solutions are mixed.
The precipitate forms because certain combinations of cations and anions produce compounds that are insoluble in water.
Solubility rules you must know
| Ion | Soluble? | Exceptions |
|---|---|---|
| All sodium / potassium / ammonium salts | Yes | None |
| All nitrates | Yes | None |
| Most chlorides | Yes | AgCl, PbCl₂ insoluble |
| Most sulfates | Yes | BaSO₄, PbSO₄, CaSO₄ insoluble |
| Most carbonates | No | Na₂CO₃, K₂CO₃, (NH₄)₂CO₃ soluble |
| Most hydroxides | No | NaOH, KOH soluble; Ca(OH)₂ slightly soluble |
Preparing an insoluble salt by precipitation
This is Method 3 of salt preparation:
- Choose two soluble compounds whose ions will combine to give the insoluble salt
- Mix the solutions — the precipitate forms immediately
- Filter to collect the precipitate
- Wash the precipitate with distilled water (removes soluble impurities)
- Dry between filter papers or in a warm oven
Example — preparing barium sulfate:
BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq)
Ionic equation: Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)
Precipitation in ion tests
Many qualitative analysis tests rely on precipitation:
- Adding NaOH(aq) to identify metal cations by their hydroxide precipitate colour
- Adding AgNO₃(aq) to identify halide ions (white AgCl, cream AgBr, yellow AgI)
- Adding BaCl₂(aq) to identify sulfate ions (white BaSO₄ precipitate)
Worked exam question
A student mixes silver nitrate solution with sodium chloride solution. (a) State the observation. [1] (b) Write the balanced equation. [1] (c) Write the ionic equation. [1]
Mark scheme
(a) A white precipitate forms [1]
(b) AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq) [1]
(c) Ag⁺(aq) + Cl⁻(aq) → AgCl(s) [1]
Common exam mistakes
- Forgetting to wash the precipitate — without this step, soluble impurities contaminate the product.
- Writing the wrong state symbol: the precipitate must be (s), not (aq).
- Choosing two insoluble reactants to make the salt — both starting compounds must be soluble so they can be mixed as solutions.
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