7.3 Preparation of salts
- Syllabus
- 0620–2026–2027
- Topic
- 7.3
- Level
- —
Learning objectives
7.3.1Preparation, separation and• Describe preparation, separation and purification of soluble salts by reaction of an acid with: (a) an alkali by titration (b) excess metal (c) excess insoluble base (d) excess insoluble carbonate7.3.2Salt solubility rules: soluble• Describe salt solubility rules: soluble sodium/potassium/ammonium salts and nitrates; chlorides except lead/silver; sulfates except barium/calcium/lead; insoluble carbonates except sodium/potassium/ammonium; insoluble hydroxides except sodium/potassium/ammonium/calcium (partial)7.3.3Hydrated substance as a substance that• Define a hydrated substance as a substance that is chemically combined with water and an anhydrous substance as a substance containing no water7.3.4Preparation of insoluble salts by• Describe preparation of insoluble salts by precipitation7.3.5Term water of crystallisation as the• Define the term water of crystallisation as the water molecules present in hydrated crystals, including CuSO4•5H2O and CoCl2•6H2O
Prepare pure crystals of a soluble salt
| Reactants | How to make a neutral salt solution | Why this method fits |
|---|---|---|
| acid + alkali | titrate with indicator to find the exact reacting volumes; repeat those volumes without indicator | both reactants are soluble, so excess alkali cannot be filtered off |
| acid + excess metal | warm the acid, add metal until some remains, then filter | excess insoluble metal ensures no acid remains |
| acid + excess insoluble base | warm the acid, add base until some remains, then filter | excess insoluble base ensures no acid remains |
| acid + excess insoluble carbonate | warm the acid, add carbonate until fizzing stops and some remains, then filter | excess insoluble carbonate ensures no acid remains |
After obtaining the pure salt solution, heat it gently to evaporate some water until the solution is saturated. Leave it to cool so crystals form, then filter the crystals, wash them with a little cold distilled water and dry them between filter papers.
| Added solid | Useful observation | Other product |
|---|---|---|
| suitable metal | bubbling becomes slower and finally stops | hydrogen |
| insoluble base | solid eventually remains and no more dissolves | water |
| insoluble carbonate | effervescence becomes slower and finally stops | carbon dioxide and water |
Do not filter after using a soluble alkali: it passes through the filter. Do not evaporate the solution to dryness, because strong heating can damage the salt and prevents well-formed crystals from growing.
Apply salt solubility rules
| Compound family | Rule in water | Exceptions |
|---|---|---|
| sodium, potassium and ammonium salts | soluble | none required |
| nitrates | soluble | none required |
| chlorides | soluble | lead(II) chloride and silver chloride are insoluble |
| sulfates | soluble | barium sulfate, calcium sulfate and lead(II) sulfate are insoluble |
| carbonates | insoluble | sodium, potassium and ammonium carbonates are soluble |
| hydroxides | insoluble | sodium, potassium and ammonium hydroxides are soluble; calcium hydroxide is partially soluble |
Read the ions in the formula, then test the strongest general rule first: a sodium, potassium or ammonium ion, or a nitrate ion, makes the compound soluble. Otherwise apply the chloride, sulfate, carbonate or hydroxide rule and its named exceptions.
AgCl is insoluble because silver is a chloride exception. K₂CO₃ is soluble because potassium overrides the usual carbonate rule. BaSO₄ is insoluble because barium is a sulfate exception.
Partially soluble is not the same as fully soluble: calcium hydroxide dissolves only to a limited extent. State the correct qualification when applying the rule.
Distinguish hydrated and anhydrous substances
| Term | Meaning |
|---|---|
| hydrated substance | contains water chemically combined within its crystal structure |
| anhydrous substance | contains no water of crystallisation |
Heating a hydrated salt can remove its water of crystallisation and form the anhydrous salt. Adding water can reverse the change for suitable salts. For example: hydrated copper(II) sulfate ⇌ anhydrous copper(II) sulfate + water.
A formula with a dot followed by water, such as ZnSO₄·7H₂O, represents a hydrated salt. The corresponding anhydrous formula is ZnSO₄.
Hydrated does not mean merely wet. Water of crystallisation is present in a fixed chemical ratio inside the crystals; surface water can be removed by drying without changing the substance's formula.
Prepare an insoluble salt by precipitation
| Step | Action | Purpose |
|---|---|---|
| 1 | choose two soluble salts whose ions supply the required insoluble salt | ensures the reacting ions can meet in solution |
| 2 | dissolve each salt separately in distilled water | makes two aqueous solutions |
| 3 | mix the solutions and stir | forms the insoluble precipitate |
| 4 | filter the mixture | collects the precipitate as the residue |
| 5 | wash the residue with distilled water | removes soluble impurities |
| 6 | dry the residue between filter papers or in a warm place | obtains a pure, dry insoluble salt |
To prepare lead(II) iodide, mix aqueous lead(II) nitrate and aqueous potassium iodide: Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) + 2KNO₃(aq). The yellow PbI₂ precipitate is the wanted product.
The net ionic change is Pb²⁺(aq) + 2I⁻(aq) → PbI₂(s). Potassium and nitrate ions remain dissolved as spectator ions.
Keep the residue, not the filtrate. An insoluble salt is collected by filtration and drying; it is not obtained by evaporating the filtrate to crystallisation.
Define water of crystallisation
Water of crystallisation is water that is chemically combined in a hydrated crystal in a fixed ratio to the salt formula units.
| Hydrated formula | Meaning |
|---|---|
| CuSO₄·5H₂O | five water molecules for each CuSO₄ formula unit |
| CoCl₂·6H₂O | six water molecules for each CoCl₂ formula unit |
On heating, a hydrated salt can lose its water of crystallisation and become anhydrous. Hydrated copper(II) sulfate is blue and becomes white anhydrous copper(II) sulfate; hydrated cobalt(II) chloride is pink and becomes blue anhydrous cobalt(II) chloride.
The dot in a hydrated formula records the fixed number of water molecules in the crystal; it does not mean that the salt is simply dissolved in liquid water or has water on its surface.