Preparing Insoluble Salts
How to prepare insoluble salts by precipitation in IGCSE Chemistry: method, ionic equations, solubility rules, and worked examples for Cambridge 0620.
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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.
Preparing insoluble salts by precipitation is a key practical technique in the Cambridge 0620 syllabus. Unlike soluble salt preparation, which uses acid reactions and crystallisation, insoluble salts are made by mixing two soluble solutions and collecting the solid that forms.
The principle
An insoluble salt cannot be made by the usual acid + base method (because it would not dissolve in the acid to form a solution). Instead, it is made by precipitation — mixing two soluble solutions that together provide the ions of the insoluble salt.
When the two solutions are mixed, the insoluble salt forms instantly as a solid precipitate.
Solubility rules you must know
To choose the right reagents, you need to know which salts are soluble and which are insoluble:
Soluble salts
- All sodium, potassium, and ammonium salts
- All nitrates
- Most chlorides (except AgCl, PbCl₂)
- Most sulfates (except BaSO₄, PbSO₄, CaSO₄)
Insoluble salts
- Most carbonates (except Na₂CO₃, K₂CO₃, (NH₄)₂CO₃)
- Most hydroxides (except NaOH, KOH, Ca(OH)₂ is slightly soluble)
- Silver chloride (AgCl)
- Lead(II) chloride (PbCl₂)
- Barium sulfate (BaSO₄)
- Lead(II) sulfate (PbSO₄)
Step-by-step method
Example: Preparing lead(II) iodide (PbI₂)
PbI₂ is an insoluble yellow salt. To make it, you need a soluble source of Pb²⁺ ions and a soluble source of I⁻ ions.
Suitable reagents:
- Lead(II) nitrate solution, Pb(NO₃)₂ (soluble — all nitrates are soluble)
- Potassium iodide solution, KI (soluble — all potassium salts are soluble)
Procedure
- Pour approximately equal volumes of lead(II) nitrate solution and potassium iodide solution into a beaker
- Stir the mixture — a bright yellow precipitate of PbI₂ forms immediately
- Filter the mixture — the yellow precipitate (PbI₂) is collected on the filter paper; the filtrate (KNO₃ solution) passes through
- Wash the precipitate on the filter paper with distilled water to remove any soluble impurities (KNO₃ and unreacted reagents)
- Dry the precipitate — either leave in a warm oven (not too hot) or press between filter papers and leave to air-dry
Equation
Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) + 2KNO₃(aq)
Ionic equation
Pb²⁺(aq) + 2I⁻(aq) → PbI₂(s)
The K⁺ and NO₃⁻ ions are spectator ions — they appear unchanged on both sides and play no part in the reaction.
More examples
| Insoluble salt | Source of cation | Source of anion | Equation |
|---|---|---|---|
| BaSO₄ (white) | BaCl₂(aq) | Na₂SO₄(aq) | BaCl₂ + Na₂SO₄ → BaSO₄ + 2NaCl |
| AgCl (white) | AgNO₃(aq) | NaCl(aq) | AgNO₃ + NaCl → AgCl + NaNO₃ |
| CaCO₃ (white) | CaCl₂(aq) | Na₂CO₃(aq) | CaCl₂ + Na₂CO₃ → CaCO₃ + 2NaCl |
| Fe(OH)₃ (brown) | FeCl₃(aq) | NaOH(aq) | FeCl₃ + 3NaOH → Fe(OH)₃ + 3NaCl |
Choosing the right reagents
The strategy: both reagents must be soluble, and when mixed, they must produce the insoluble salt you want.
Step 1: Identify the cation and anion in the desired insoluble salt Step 2: Find a soluble salt containing the cation (nitrates are always soluble — a good default choice) Step 3: Find a soluble salt containing the anion (sodium and potassium salts are always soluble — good default choices) Step 4: Check that the other product (the salt left in solution) is also soluble
Example: How would you make barium sulfate?
- Need: Ba²⁺ and SO₄²⁻
- Source of Ba²⁺: barium chloride, BaCl₂ (soluble)
- Source of SO₄²⁻: sodium sulfate, Na₂SO₄ (soluble)
- Other product: NaCl (soluble — confirmed)
Why washing is important
After filtering, the precipitate is contaminated with the solution it was formed in (which contains dissolved soluble salts). Washing with distilled water removes these impurities. Using too much wash water can dissolve some of the precipitate if it has slight solubility — use small volumes.
Precipitation vs other methods
| Method | Used for | Key step |
|---|---|---|
| Precipitation | Insoluble salts | Mix two soluble solutions; filter the precipitate |
| Acid + excess base/metal | Soluble salts (insoluble reagent) | Add excess; filter; crystallise |
| Titration | Soluble salts (soluble alkali) | Find exact volumes; evaporate and crystallise |
Worked exam question
Describe how you would prepare a pure, dry sample of silver chloride (AgCl) starting from silver nitrate and sodium chloride. [4]
Mix silver nitrate solution with sodium chloride solution [1]. A white precipitate of silver chloride forms [1]. Filter the mixture to collect the precipitate on filter paper [1]. Wash the precipitate with distilled water and dry in a warm oven / between filter papers [1].
Write the ionic equation for this reaction. [1]
Ag⁺(aq) + Cl⁻(aq) → AgCl(s) [1]
Common exam mistakes
- Trying to make an insoluble salt by dissolving it in acid — insoluble salts do not dissolve, so acid-base methods do not work. Use precipitation.
- Choosing an insoluble reagent to provide the ions — both starting solutions must be soluble, otherwise they cannot provide free ions for the reaction.
- Forgetting to wash the precipitate — without washing, the product is contaminated with the other soluble salt from the reaction.
- Not knowing the solubility rules — you must be able to identify which salts are soluble and which are insoluble to select appropriate reagents.
- Writing the ionic equation incorrectly — only include the ions that form the precipitate; leave out spectator ions.
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