Practical 2

Crystallisation

You will carry out crystallisation to obtain hydrated copper (II) sulfate crystals, CuSO4.5H2O from its solution.

Warnings

Copper (II) sulfate can be hazardous. Do not touch copper (II) sulfate with your bare hands.

Theory

Crystallisation is used to obtain a pure solid from its saturated solution. A saturated solution is one in which no more solute can be dissolved. Crystallisation is a gentler method that can be used to separate solutes (dissolved solids) that are less heat-resistant from its solution. These solutes may decompose (break down into simpler substances) when heated strongly.

For most substances, solubility increases with temperature. Hence, when a saturated solution cools down, the copper (II) sulfate becomes less soluble, resulting in crystallisation.

Apparatus provided:

Evaporating dish

Tripod stand

Wire gauze

Lighter

Bunsen burner

Fig. 1

Instructions

1. Fig. 1 shows how the apparatus is arranged.

2. Fill about half to 3-quarter of the evaporating dish with copper (II) sulfate solution.

3. Heat the copper (II) sulfate solution until half its volume, to obtain a saturated solution.

4. Allow the hot saturated copper (II) sulfate solution to cool and crystallise. (Do not disturb the solution during cooling for nicer crystals)

Steps we will not carry out:

5. Filter the saturated copper (II) sulfate solution and use a spatula to obtain the copper (II) sulfate crystals.

6. Wash the crystals with a little bit of cold distilled water.

7. Dry the crystals between sheets of filter paper.

Actual Questions

1. Suggest how you would test if the solution is saturated.

2. Why is the solution heated during the experiment?

3. Why should the solution not be boiled vigorously during heating?

4. State the colour of hydrated copper (II) sulfate and that of anhydrous copper (II) sulfate.

5. Can crystallisation be used to separate heat-resistant solutes (such as salt) from its solution?

Extension Questions for Research

1. What steps during the experiment can we take to ensure that the crystals obtained are pure?

2. Why is it important to leave the solution undisturbed during cooling?

3. Is crystallisation or evaporation to dryness a better method for separating heat-resistant solutes from its solution? Weigh the pros and cons.

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