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Cambridge IGCSE Chemistry · 0620

Chapter 7: Acids, Bases and Salts — Part 3

Topic 7.3 · Preparation of salts

Solubility Rules for Salts

To prepare a salt, its solubility must be identified:

Type of Salt Solubility
Sodium, potassium, and ammonium salts Soluble
Nitrates Soluble
Chlorides Soluble (except lead and silver chloride)
Sulfates Soluble (except barium, calcium and lead sulfate)
Carbonates Insoluble (except sodium, potassium and ammonium)
Hydroxides Insoluble (except sodium, potassium and ammonium; calcium is partially soluble)

Exam Traps

  • Do not say all chlorides are soluble — AgCl and PbCl2 are insoluble.
  • Do not say all sulfates are soluble — BaSO4, CaSO4, and PbSO4 are insoluble.

Preparation of Soluble Salts

  • From an Insoluble Reactant (Metal, Base, or Carbonate):
    1. Add the insoluble reactant to heated dilute acid until it is in excess (no more dissolves).
    2. Filter the mixture to remove the excess solid.
    3. Heat the filtrate in an evaporating dish until saturated, then allow it to cool so crystals form.
  • From an Alkali (Titration):
    1. Use a titration to determine the exact volumes of acid and alkali needed for neutralisation.
    2. Mix these volumes without an indicator to get a pure salt solution.
    3. Evaporate the water and crystallise the salt.
Diagram showing the filtration of excess base and the subsequent crystallisation of the salt solution
Diagram 1: Diagram showing the filtration of excess base and the subsequent crystallisation of the salt solution

Exam Traps

  • Do not use the titration mixture containing indicator for the final salt — repeat the titration without indicator.

Preparation of Insoluble Salts

  • Insoluble salts are prepared by precipitation.
  • Method: Mix two soluble salts in aqueous solution; the insoluble salt forms a precipitate. The precipitate is filtered, washed with distilled water, and dried in an oven.

Hydration and Water of Crystallisation

  • A hydrated substance is chemically combined with water (e.g., blue CuSO4·5H2O), while an anhydrous substance contains no water (e.g., white CuSO4).
  • Water of crystallisation refers to the water molecules present in hydrated crystals, often indicated by a dot in the formula (e.g., CoCl2·6H2O contains 6 molecules of water of crystallisation).

Exam Traps

  • Do not treat the dot as ordinary mixture — water of crystallisation is part of the crystal structure.
  • Do not confuse hydrated (with water of crystallisation) with aqueous (dissolved in water).

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