1. A salt can be made by reacting an acid with an insoluble base.

A student has a sample of copper(Il) oxide.

The student uses this method.

• Stage 1 pour 50 cm³ of dilute sulfuric acid into a beaker

• Stage 2 warm the acid using a Bunsen burner

• Stage 3 add a small amount of copper(II) oxide to the warm acid and stir the mixture

• Stage 4 add further amounts of copper(II) oxide until copper(II) oxide is in excess

• Stage 5 filter the mixture

• Stage 6 obtain crystals from the filtrate

State why the acid is warmed in stage 2.