Making soluble salts

A soluble salt can be made by reacting an acid with an insoluble metal, metal oxide, metal hydroxide or metal carbonate.

The solid is added in excess — more than is needed — so that all the acid reacts. The leftover solid is then removed by filtering, leaving a solution of just the salt. Finally, the salt is crystallised out of the solution.

For copper sulfate:

copper oxide + sulfuric acid → copper sulfate + water

CuO + H₂SO₄ → CuSO₄ + H₂O

1. Gently warm about 40 cm³ of dilute sulfuric acid in a beaker (don't boil it).
2. Stir in copper(II) oxide a spatula at a time until some black solid stays undissolved: the oxide is in excess, so all the acid has reacted.
3. Filter the mixture to remove the excess copper oxide. The blue filtrate is copper sulfate solution.
4. Heat the filtrate in an evaporating basin over a water bath until crystals start to form at the edge.
5. Leave it in a warm place for a day or two to crystallise.
6. Remove the blue crystals and pat them dry with filter paper.

Hazard: sulfuric acid is an irritant and copper compounds are harmful — wear eye protection and don't heat to dryness, which can make the solution spit.

Better results: heat gently and let the solution cool slowly to get larger, purer crystals.