Insoluble SaltsOxford AQA IGCSE Chemistry: Revision notes

Not all salts dissolve in water. Insoluble salts are made by precipitation — mixing two solutions whose ions combine to form a solid that will not dissolve.

• Two soluble salt solutions are chosen so that when mixed, the positive ion from one and the negative ion from the other combine to form an insoluble compound
• The insoluble solid formed is called a precipitate
• The reaction is called a precipitation reaction

To make a named insoluble salt by precipitation, you must choose two soluble solutions that each contain one of the required ions.

1. Identify the positive ion and negative ion needed in the target salt
2. Choose a soluble solution containing the positive ion
3. Choose a soluble solution containing the negative ion
4. Mix the two solutions together

Students should be able to name suitable starting solutions for a given insoluble salt.

Once the precipitate has formed, it must be separated from the solution.

• Filter the mixture — the precipitate is collected in the filter paper as the residue, while the soluble by-product passes through as the filtrate
• Wash the precipitate with distilled water to remove any remaining soluble impurities
• Dry the precipitate, e.g. between sheets of filter paper or in a warm oven

Precipitation reactions are useful beyond the laboratory. They can be used to remove unwanted ions from solution.

• Used in treating drinking water, to remove harmful ions
• Used in treating effluent (waste water), to remove pollutant ions before the water is released

In both cases, the unwanted ion is precipitated out as an insoluble solid and then filtered off.