Preparing a soluble salt solution from an acid and an insoluble solid

A soluble salt solution can be made by reacting an acid with a solid that does not dissolve in water. The solid must react with the acid to form the required salt. Suitable solids include some metals, metal oxides, metal hydroxides and metal carbonates.

For example, copper oxide reacts with sulfuric acid to form copper sulfate and water: $$\ce{CuO + H2SO4 -> CuSO4 + H2O}$$. The copper sulfate dissolves in the water, forming a solution.

Preparing the salt solution

1. Warm the acid if needed, then add the chosen solid a little at a time, stirring as it reacts.
2. Continue adding the solid until no more reacts and some solid remains. The solid is now in excess, showing that all the acid has been used up.
3. Filter the mixture. The unreacted solid stays on the filter paper as the residue, while the clear salt solution passes through as the filtrate.

Adding excess solid is important because it leaves no unreacted acid to contaminate the solution. This method works when the added solid is insoluble: any excess remains solid and can therefore be separated from the soluble salt solution by filtration.

Crystallising a soluble salt from solution

A soluble salt can be recovered from its solution as solid crystals. Crystallisation works by removing some water and then cooling the solution, so that some of the dissolved salt comes out of solution as a solid.

1. Heat the salt solution gently so that some water evaporates. The salt remains in the solution, which becomes more concentrated. Stop heating before all the water has evaporated.
2. Leave the concentrated solution to cool. Crystals of the salt form as dissolved salt comes out of the solution.
3. Separate the crystals from the remaining solution by filtration, then dry them, for example between sheets of filter paper.

The result is a pure, dry sample of the soluble salt. Some salt may remain dissolved in the liquid left after the crystals have formed.