IGCSE Chemistry 0620: Cobalt(II) Chloride Test and Freezing Point for Water Purity

A pharmaceutical company buys in bulk deliveries of water from an external supplier for use in a laboratory. Before each delivery is accepted, a quality-control technician runs simple checks on a sample of the water.

(a) The technician touches a strip of blue anhydrous cobalt(II) chloride paper into a drop of the water sample. State the colour change that would show her that water is present in the sample. [1]

(b) The technician then slowly cools a portion of the same water sample in a freezer and records the exact temperature at which it starts to freeze. Pure water freezes at exactly $0\,^\circ\text{C}$ at standard atmospheric pressure. State what it would mean about the sample if its freezing point were recorded as below $0\,^\circ\text{C}$ , and explain why. [2]

(c) Explain why the cobalt(II) chloride test in part (a), on its own, would NOT be a reliable way to decide whether this delivery of water is pure enough to accept. [2]

Part (a): Testing for the presence of water

Anhydrous cobalt(II) chloride is a standard chemical test for the presence of water, used as an alternative to anhydrous copper(II) sulfate. It starts as a blue solid (often used as blue cobalt(II) chloride paper).

When water is added, it forms hydrated cobalt(II) chloride, which is pink.

So the technician would see the paper turn from blue to pink.

Part (b): Testing for purity using freezing point

Pure water has a fixed, sharp freezing (melting) point of exactly $0\,^\circ\text{C}$ at standard atmospheric pressure. Dissolved impurities lower this freezing point below $0\,^\circ\text{C}$ . This is the opposite effect to their effect on boiling point, which they raise above $100\,^\circ\text{C}$ .

So if the sample’s freezing point were recorded as below $0\,^\circ\text{C}$ , this would mean the water sample is impure. It contains dissolved substances that have depressed the freezing point below that of pure water.

Part (c): Why the cobalt(II) chloride test cannot confirm purity

The cobalt(II) chloride test only detects the presence of water. It turns from blue to pink whenever water is present, regardless of whether that water is pure or contains dissolved impurities. Both a pure water sample and a heavily contaminated one would give exactly the same colour change, so this test cannot be used to judge purity.

To check purity, the technician needs a test that is sensitive to dissolved impurities, such as measuring the melting/freezing point or boiling point, since these shift measurably away from the pure-water values ( $0\,^\circ\text{C}$ and $100\,^\circ\text{C}$ ) when impurities are present.

Final answers

(a) Blue anhydrous cobalt(II) chloride turns pink.
(b) A freezing point below $0\,^\circ\text{C}$ shows the sample is impure, because dissolved impurities lower the freezing point below that of pure water.
(c) The cobalt(II) chloride test only confirms water is present, not that it is pure, since it gives the same result for pure and impure water; a purity test (freezing point or boiling point) is needed instead.