Making salts of different solubility

Chemistry - Acids, bases and salts - Salts of different solubility

This post is about how to make salts of different solubility through mixing compounds. Different methods are required for different levels of solubility and there are three categories namely: Insoluble salts, Soluble salts of low reactivity and Soluble salts of high reactivity. 

Insoluble salts

To make a pure sample of an insoluble salt, a precipitation reaction is used. A precipitate is a fine solid that is formed by a chemical reaction involving liquids or gases.

Two ionic solutions come together. In either of the pairs the attractions aren’t strong enough for the ions to stick together but now a pair is formed through strong bonds – the precipitate. The other ions have not changed at all, and are therefore spectator ions.

To choose what to mix, take one solution that has the appropriate positive ion and one solution that contains the negative ion.

To claim the salt, ‘filter, wash and dry the precipitate’.

Making Sodium, Potassium and Ammonium salts

This is why you can’t add an excess solid to a solution: the solid would react with the solution, but the excess would just dissolve in the water present, making it impossible to filter.

This problem is solved by doing a titration. Usually, sodium, potassium hydroxide and ammonia solution would be used, but their carbonates can also be used. These solutions are alkaline so the end-point can be determined with an indicator. Usually methyl orange is used. Yellow indicates too little acid and red too much, making orange the endpoint.

The experiment is run twice with indicator to find the correct volumes of each, and then it is performed again without the indicator with the correct volumes.

Finish by evaporating the water until crystals form upon cooling, and then leave the solution to crystallise on its own.

Making other soluble salts

This technique is used with

·       Acid + metal

·       Acid + metal oxide or hydroxide

·       Acid + carbonate

You add the solid to the acid in excess to ensure all the acid is used up, and then filter off the excess solid. The rest is heated until crystals form upon cooling.

You will usually heat the reaction unless it is a carbonate or magnesium.