Making insoluble salts (Edexcel GCSE Chemistry): Revision Notes

A precipitation reaction occurs when two soluble salts react together to form an insoluble salt that precipitates out of the solution. This is a common method for making insoluble salts in the lab.

Example: Making Lead Chloride (PbCl₂)

We will use lead nitrate $(Pb(NO₃)₂)$ and sodium chloride (NaCl) as the two soluble salts to make lead chloride, an insoluble salt.

Method: How to Make a Pure, Dry Sample of Lead Chloride

1. Prepare Lead Nitrate Solution:

• Add 1 spatula of lead nitrate to a test tube.
• Add some deionised water (use deionised water to prevent unwanted ions from interfering).
• Shake the test tube to ensure the lead nitrate dissolves fully.

2. Prepare Sodium Chloride Solution:

• In a separate test tube, add 1 spatula of sodium chloride.
• Add deionised water and shake to dissolve the sodium chloride.

3. Mix the Solutions:

• Pour both solutions into a small beaker.
• Stir the mixture well. A white precipitate of lead chloride should form.

4. Filter the Mixture:

• Set up a philtre funnel with filter paper over a conical flask.
• Pour the contents of the beaker into the philtre, catching the solid lead chloride in the philtre paper.
• Rinse the sides of the beaker with deionised water and add to the philtre to ensure all the precipitate is collected.

5. Rinse the Precipitate:

• Rinse the solid lead chloride on the philtre paper with deionised water to remove any soluble sodium nitrate (NaNO₃).

6. Dry the Lead Chloride:

• Carefully scrape the lead chloride onto fresh philtre paper.
• Leave it to dry in a warm oven or use a desiccator to remove any remaining water.

Key Points to Remember:

• Use deionised water to avoid adding extra ions to the reaction.
• Filter the mixture to separate the insoluble salt (precipitate) from the rest of the solution.
• Rinse the precipitate to remove any remaining soluble substances.
• Dry the precipitate thoroughly to obtain a pure sample.