Soluble salts can be made by reacting an acid with an insoluble solid: a metal, a metal oxide, a metal hydroxide or a metal carbonate. The solid is added to the acid until no more reacts. You know this has happened when some solid is left over, or when bubbles stop forming with a carbonate. The solid is then in excess, which means all of the acid has been used up. The leftover solid is removed by filtering, which leaves a solution of the salt.
Required practical 1 makes a pure, dry sample of copper sulfate from copper oxide and dilute sulfuric acid:
- Warm the dilute acid in a beaker, using a Bunsen burner.
- Add the copper oxide a little at a time and stir, until the black powder stops disappearing.
- Filter the mixture. The filtrate is a blue copper sulfate solution.
- Pour the filtrate into an evaporating basin and evaporate some of the water gently, using a water bath or an electric heater. Stop when crystals start to form at the edge, the point of crystallisation.
- Leave the solution to cool so that the salt can crystallise. Filter off the crystals and dry them with filter paper or in a warm oven.
Gentle heating is used so the solution does not spit and the crystals are not damaged. The method is the same for a carbonate, for example copper carbonate, but you also see fizzing as carbon dioxide is released.
