Preparing Insoluble Salts (Cambridge International IGCSE Chemistry)

Spec point — 7.2.2 · Describe the preparation of insoluble salts by precipitation

Preparing insoluble salts

Extended tier only

• Insoluble salts can be prepared using a precipitation reaction

• The solid salt obtained is the precipitate

• To successfully use this method:

  • The solid salt (precipitate) being formed must be insoluble in water
  • The reactants must be soluble

Using two soluble reactants

Diagram showing the filtration of the mixture to remove the precipitate

Method

• Make aqueous solutions of two soluble salts
• Mix together using a stirring rod in a beaker
• Filter to remove the precipitate from the mixture
• Wash the precipitate with distilled water to remove traces of other solutions
• Leave to dry (for example in a warm oven)

Example: Preparation of pure, dry, solid lead(II) sulfate using a precipitation reaction

• Dissolve lead(II) nitrate and potassium sulfate in water
• Mix together using a stirring rod in a beaker
• Filter to remove the precipitate from the mixture
• Wash the precipitate with distilled water to remove traces of potassium nitrate solution
• Leave to dry (for example in a warm oven)

lead(II) nitrate + potassium sulfate → lead(II) sulfate + potassium nitrate

Pb(NO₃)₂(aq) + K₂SO₄(aq) → PbSO₄(s) + 2KNO₃(aq)