Crystals of copper sulfate are prepared by reacting copper oxide, a base, with dilute sulfuric acid - Edexcel - GCSE Chemistry - Question 8 - 2023 - Paper 1

The student knows to stop adding copper oxide when an observation is made that the black powder has completely disappeared from the mixture. This indicates that the copper oxide has reacted fully with the dilute sulfuric acid to form copper sulfate, which is visible as a pale blue solution. Additionally, the student may notice that there is an excess of copper oxide present at this point, thus stopping further addition will prevent wastage.

A. pour the solution through filter paper in a funnel