Insoluble Salts (Oxford AQA IGCSE Combined Science Double Award)

Making Insoluble Salts

• Insoluble salts can be prepared using a precipitation reaction

• Precipitation can be used to remove unwanted ions from solutions

  • E.g. Treating water for drinking or in treating effluent

• The solid salt obtained is the precipitate

Using two soluble reactants to prepare an insoluble salt

 Method:

• Dissolve soluble salts in water and mix together using a stirring rod in a beaker

• Filter to remove precipitate from mixture

• Wash filtrate with  distilled water to remove traces of other solutions

• Leave in an oven to dry

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

• Soluble salt 1 = lead(II) nitrate        

• Soluble salt 2 = potassium sulfate

Method:

1. Dissolve lead(II) nitrate and potassium sulfate in water and mix together using a stirring rod in a beaker

2. Filter to remove precipitate from mixture

3. Wash precipitate with distilled water to remove traces of potassium nitrate solution

4. Leave in an oven to dry

Equation of reaction:

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

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