IGCSE Chemistry 0620: Testing Rainwater for Presence, Purity and Treatment

An outdoor education centre on the shore of Silverlynn Lake collects rainwater in a butt behind the boathouse to top up the freshwater aquarium tanks used in its marine biology classes. Before the water is used, a technician runs some simple checks on a sample.

(a) The technician places a few drops of the rainwater onto white anhydrous copper(II) sulfate powder. State the colour change she would see if the powder confirms that the liquid contains water. [1]

(b) The technician then heats a sample of the rainwater and finds that it boils at exactly $100\,^\circ\text{C}$ at standard atmospheric pressure. State what this result shows about the purity of the rainwater, and explain why a boiling point test, rather than the colour-change test in (a), is used to check the purity of water. [2]

(c) Before reaching the header tank, the rainwater passes through the centre's own small treatment unit. State the purpose of each of the following stages in this unit: (i) sedimentation and filtration (ii) passing through a carbon filter (iii) chlorination [3]

Part (a): Testing for the presence of water

Anhydrous copper(II) sulfate is a standard chemical test for the presence of water. It starts as a white powder.

When water is added, it reacts to re-form hydrated copper(II) sulfate, which is blue:

$\text{CuSO}_4(s) + 5\text{H}_2\text{O}(l) \rightarrow \text{CuSO}_4\cdot 5\text{H}_2\text{O}(s)$

So the technician would see the powder turn from white to blue.

Part (b): Testing for the purity of water

A test for purity is different from a test for presence. Pure water has a fixed, sharp boiling point of exactly $100\,^\circ\text{C}$ at standard atmospheric pressure. Dissolved impurities raise the boiling point above this value.

Since the sample boiled at exactly $100\,^\circ\text{C}$ , this shows the rainwater is (essentially) pure. It contains no significant dissolved impurities that would have pushed the boiling point higher.

Boiling point (or melting point) is used to test purity, rather than the copper(II) sulfate test in part (a), because:

The copper(II) sulfate test only shows whether water is present at all. It turns blue whether the water is pure or full of dissolved impurities.
The boiling point test is sensitive to dissolved impurities: any impurity present would shift the boiling point away from exactly $100\,^\circ\text{C}$ , so it can distinguish pure water from impure water.

Part (c): Purpose of each treatment stage

(i) Sedimentation and filtration: these remove insoluble suspended solids (e.g. grit, dust, dead leaves) from the water.

(ii) Carbon filter: this removes unwanted tastes and odours from the water.

(iii) Chlorination: this kills harmful microbes in the water, making it safe (disinfection).

Final answers

(a) White anhydrous copper(II) sulfate turns blue.
(b) Boiling at exactly $100\,^\circ\text{C}$ shows the rainwater is pure; boiling point is used (rather than the copper(II) sulfate test) because dissolved impurities would raise the boiling point, so it tests purity directly rather than just presence of water.
(c) (i) Removes insoluble solids. (ii) Removes tastes and odours. (iii) Kills harmful microbes.