Fig. 8.1 A student prepared blue copper(II) sulfate crystals by reacting warm dilute sulfuric acid with an excess of copper(II) oxide, an insoluble base. Th...

This is the standard preparation of a soluble salt from an acid and an insoluble base. Because copper(II) oxide will not dissolve unless it reacts, adding it in excess is the trick that lets you remove all the leftover acid.

(a) The reaction between a base (copper(II) oxide) and an acid is neutralisation [1]:

(b) Preparing pure, dry crystals (any five points, max [5]):

1. Warm the dilute sulfuric acid.
2. Add copper(II) oxide a little at a time, stirring, until no more dissolves and some solid remains (excess).
3. Filter to remove the excess copper(II) oxide, leaving a blue copper(II) sulfate solution.
4. Heat the filtrate to evaporate some of the water to the point of crystallisation.
5. Leave the solution to cool slowly so that crystals form.
6. Remove the crystals and dry them between sheets of filter paper.

(c) The excess copper(II) oxide is added to make sure all the acid has reacted / is used up, so no unreacted acid contaminates the product [1].

(d) The unreacted copper(II) oxide is separated using a filter funnel with filter paper [1] by the process of filtration [1]. The insoluble oxide stays on the paper as residue while the dissolved salt passes through as the filtrate. Do not heat the crystals strongly to dry them, or you drive off the water of crystallisation and turn the blue crystals white.