Hydrated copper(II) sulfate forms blue crystals. A student is given a warm, dilute aqueous solution of copper(II) sulfate and is asked to obtain a good samp...

This question tests the crystallisation method used to obtain good crystals of a soluble salt, and the idea that solubility falls as a solution cools.

(a) Why heat off about half the water first [2]. Removing water makes the remaining solution more concentrated, closer to saturated [1], so that crystals will actually form when it is later cooled [1]. A dilute solution could be cooled a long way without depositing any solid.

(b) What happens as the solution cools [2]. The solubility of copper(II) sulfate falls as the temperature drops [1], so the excess solid can no longer stay dissolved and comes out of solution as crystals [1]. Slow cooling gives larger, well formed crystals.

(c) Separating crystals from the liquid [1]. The crystals are filtered off from the remaining solution (filtration); decanting is also acceptable [1].

(d) Drying the crystals [1]. Dab or press them gently between sheets of filter paper, or leave them in a warm place to dry [1]. Strong heating is avoided so the crystals are not damaged.

(e) Why not heat to dryness [2]. Heating all the way to dryness would drive off the water of crystallisation (or decompose the crystals) [1], and would leave only a mass of tiny powdery crystals instead of good regular ones [1]. The blue colour and shape depend on that trapped water.

(f) Colour of the crystals [1]. Hydrated copper(II) sulfate crystals are blue [1].

Exam tip: for crystallisation always state that solubility decreases on cooling, and that you stop heating at the point of saturation (a hot saturated solution), never taking it to dryness.