Precipitation Reaction

The aim of this experiment is to prepare insoluble salt from aqueous solutions of two salts. This reaction is also called a precipitation reaction.

A reaction in which two salts combine to form an insoluble salt called a precipitate is known as a precipitation reaction. 

When a solution of sodium sulphate is mixed with barium chloride solution, a double displacement reaction takes place. 

                  Na₂SO₄ (aq) + BaCl₂ (aq) ? BaSO₄ (s) + 2NaCl (aq)

In this reaction, sulphate ions and chloride ions are exchanged between the reactants. The sulphate ions displace the chloride ions from barium chloride to form barium sulphate, which appears as a white precipitate. Similarly, the chloride ions displace sulphate ions from sodium sulphate to form sodium chloride, which remains in the solution. Thus, this chemical reaction is an example of a double displacement reaction.

To perform this experiment, apparatus and chemicals are required: test tubes, chemicals, Na₂SO₄ solution, BaCl₂ solution, KI solution, and Pb(NO₃)₂ solution.

To prepare insoluble salt, we have to follow the given procedure.

1. Take 3 mL of sodium sulphate solution in a test tube labelled ‘A’. 
2. In another test tube labelled ‘B’, take 3 mL of barium chloride solution. 
3. Observe the colour of the solutions in test tubes ‘A’ and ‘B’.
4. Transfer the solution from test tube ‘A’ to test tube ‘B’. 
5. Mix the two solutions with gentle shaking.
6. Observe the changes in the solution in the test tube.
7. A white insoluble solid is formed in the test tube. 
8. Now, take lead nitrate and potassium iodide solution in two different test tubes. 
9. Pour the contents from any one of the test tubes into the other. 
10. A yellow precipitate is observed when mixing the two solutions. 

In this experiment, we learnt that: 

• A reaction in which two salt solutions combine to form an insoluble salt is called a precipitation reaction.
• The insoluble salt formed in the reaction is called a precipitate.
• The chemical equation for the reaction is: 

BaCl₂ (aq) + Na₂SO₄ (aq) ? BaSO₄ (s) + 2NaCl (aq) 

• BaSO₄ formed in the reaction appears as a white precipitate, but NaCl remains dissolved in the solution.

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