Preparing Insoluble Salts
Section: 7. Acids, Bases & Salts | Syllabus: Cambridge IGCSE Chemistry (0620)
Precipitation
An insoluble salt cannot be made by the acid-based methods (it wouldn't dissolve to be crystallised). Instead, mix two soluble solutions that each supply one of the required ions - the insoluble salt forms immediately as a precipitate.
soluble salt A(aq) + soluble salt B(aq) → insoluble salt(s) + soluble salt(aq)
- Mix the two soluble solutions - the precipitate forms at once
- Filter to collect the precipitate
- Wash with distilled water to remove soluble impurities
- Dry between filter paper or in a warm oven (no crystallisation needed)
Worked Example: Lead(II) Chloride
- Question: which two compounds react together to form the insoluble salt lead(II) chloride, given: 1 - lead(II) nitrate (soluble), 2 - lead(II) sulfate (insoluble), 3 - silver chloride (insoluble), 4 - sodium chloride (soluble)? [1]
- Answer: 1 and 4. Both reactants must be soluble so they can be mixed as solutions: Pb(NO₃)₂(aq) + 2NaCl(aq) → PbCl₂(s) + 2NaNO₃(aq)
Ionic equation: Pb²⁺(aq) + 2Cl⁻(aq) → PbCl₂(s) - the Na⁺ and NO₃⁻ are spectator ions and don't appear.
Choosing Soluble Reactants
Nitrates are always soluble, so a metal nitrate is a safe soluble source of a cation. Sodium/potassium salts are always soluble, so they are a safe soluble source of an anion.
Worked Example: Planning a Precipitation
- Question: name two soluble compounds that would react to form barium sulfate. [2]
- Answer: a soluble barium compound, e.g. barium chloride/nitrate, and a soluble sulfate, e.g. sodium sulfate (or dilute sulfuric acid): BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq)
Mix two soluble solutions, filter out the precipitate, wash, then dry - no crystallisation step is needed
Common Mistakes
- ❌ Trying to use an insoluble compound as a "soluble source" of an ion
- ✓ both starting solutions must actually dissolve, otherwise you can't mix them as solutions in the first place - check the solubility rules before choosing reactants
- ❌ Evaporating/crystallising an insoluble salt like a soluble one
- ✓ insoluble salts are already solid the moment they form - just filter, wash, and dry (no heating/evaporating solution needed)
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