Preparing soluble salts (general method)
The general method is the same as the required practical (C4.5): pick an acid that gives the right anion, an insoluble base that gives the right cation, react them, filter, crystallise.
Choosing the right reactants
- For a chloride salt, use HCl.
- For a sulfate salt, use H₂SO₄.
- For a nitrate salt, use HNO₃.
The base provides the metal. The base must be insoluble to allow the "excess" trick (so the unreacted excess can be filtered off).
✦Worked example— Examples
- Magnesium sulfate: H₂SO₄ + MgO (or Mg(OH)₂ or MgCO₃) → MgSO₄ + H₂O (+ CO₂ if carbonate).
- Zinc nitrate: 2HNO₃ + ZnO → Zn(NO₃)₂ + H₂O.
- Iron(III) chloride: 6HCl + Fe₂O₃ → 2FeCl₃ + 3H₂O.
Method (universal)
- Warm the acid in a beaker (gentle heat speeds the reaction).
- Add excess insoluble base while stirring, until no more dissolves.
- Filter to remove unreacted base.
- Pour filtrate into evaporating basin.
- Heat over water bath until volume reduced and saturated.
- Leave to cool slowly — large crystals form.
- Filter and pat dry.
The product is a pure, dry, crystalline soluble salt.
Why solubility decides the method
Group 1 metal (e.g. Na, K) hydroxides and carbonates are soluble — adding them in excess gives a solution, not a filterable solid. So for sodium/potassium/ammonium salts, use titration (acid + soluble alkali, with an indicator) — see C4.7.
For other metals (Cu, Zn, Fe, Mg, Pb, Ca etc.), the hydroxide/carbonate/oxide is insoluble: the excess-and-filter method (C4.5) works.
✦Worked example— Worked example — making zinc chloride
Aim: prepare crystals of ZnCl₂.
- Choose: HCl (gives chloride) + ZnO (insoluble) or ZnCO₃ (insoluble).
- React: ZnO + 2HCl → ZnCl₂ + H₂O.
- Filter, evaporate over water bath, cool, crystallise.
⚠Common mistakes
- Picking a soluble base (NaOH) for this method — won't work.
- Forgetting to balance the equation, especially when the metal is 2+ or 3+.
- Boiling dry the solution instead of slow crystallisation — gives a powder, not crystals.
- Skipping the filter step — gives contaminated salt.
Links
Same procedure as C4.5. Pairs with C4.7 (titration for soluble bases). The method explains why we choose certain reagents in industrial salt production.
AI-generated · claude-opus-4-7 · v3-deep-chemistry