A student is preparing a sample of copper sulfate crystals in a school laboratory. They dissolve excess copper oxide in dilute sulfuric acid, then use an appropriate separation technique to obtain dry copper sulfate crystals from the resulting solution. Explain why crystallisation is the most appropriate separation technique to use in this situation, and describe how the student would carry out crystallisation to obtain dry copper sulfate crystals.

Crystallisation is the most suitable method because copper(II) sulfate is a highly soluble solid that must be separated from the aqueous medium.

1. Dissolve the excess copper(II) oxide in dilute sulphuric acid to give a clear copper(II) sulfate solution.
2. Heat the solution in an evaporating dish until it becomes saturated and crystals begin to appear.
3. Allow the hot solution to cool slowly; as the temperature falls the solubility decreases and more crystals grow.
4. Once the solution is cold, filter the suspension to collect the crystals.
5. Pat the crystals dry with a paper towel or leave them to air‑dry at room temperature (or in a low‑temperature oven) until all water is removed.