Choose the method from the salt, not from habit. Decide first whether the salt is soluble in water, then pick the route that gives it.
This lesson is part of SPM Chemistry acids, bases and salts. The equations that follow from each method are in writing neutralisation equations.
How do I pick the method?
Use solubility as the first question. The rules on your syllabus give the answer.
| Salt type | Method | Key idea |
|---|---|---|
| Sodium, potassium or ammonium salt | Acid + alkali, titration | Both reactants are solutions, so the exact volume is found |
| Other soluble salt (most nitrates, sulfates, chlorides) | Acid + insoluble base, carbonate or reactive metal | Add the solid in excess, filter, crystallise |
| Insoluble salt | Mix two soluble salt solutions | The salt forms as a solid, so filter, rinse, dry |
Remember the common exceptions: lead(II), barium and calcium sulfates are insoluble, and lead(II) and silver chlorides are insoluble.
Worked example: copper(II) sulfate
Copper(II) sulfate is soluble, and copper does not react with dilute acid. The route is dilute sulfuric acid with copper(II) oxide.
- Warm the dilute sulfuric acid gently.
- Add copper(II) oxide a little at a time, stirring, until some solid no longer dissolves.
- Filter to remove the excess copper(II) oxide.
- Heat the filtrate to evaporate about half the water, until a drop on a cold glass rod forms crystals.
- Leave the hot solution to cool and crystallise.
- Filter the crystals, rinse with a little cold distilled water, and press dry between filter papers.
The equation is CuO + H₂SO₄ → CuSO₄ + H₂O.
The contrast: an insoluble salt
Lead(II) sulfate is insoluble, so acid plus a solid would coat the lead and stop. Mix lead(II) nitrate solution with sodium sulfate solution instead. A white precipitate forms.
Filter it, rinse with distilled water to remove the sodium nitrate solution, and dry on filter paper. The equation is Pb(NO₃)₂ + Na₂SO₄ → PbSO₄ + 2NaNO₃.
The mistake that loses marks
The common slip is using copper with dilute sulfuric acid, because “metal plus acid gives salt” is a familiar rule. Copper is below hydrogen in the reactivity series, so nothing happens.
Check that the metal reacts before you choose this route. If it does not, use the metal oxide or carbonate.
Check yourself
Plan how to prepare a dry sample of barium sulfate in the laboratory. State the method and name the solutions.
Answer
Barium sulfate is insoluble, so use double decomposition. Mix barium nitrate solution with sodium sulfate solution and stir. A white precipitate of barium sulfate forms.
Filter it, rinse with distilled water, and dry between filter papers. Equation: Ba(NO₃)₂ + Na₂SO₄ → BaSO₄ + 2NaNO₃.
What to study next
Turn the steps into quantities with calculating acid-base quantities. The mole and stoichiometry steps tool helps you check the amounts in a preparation.
To have a teacher go through your planned methods with you, see online one-to-one Chemistry tuition.