Water: Chemical Tests
Section: 10. Chemistry of the Environment | Syllabus: Cambridge IGCSE Chemistry (0620)
Chemical Tests for the Presence of Water
Two anhydrous salts change colour when water is added to them, so they are used as simple chemical tests for the presence of water:
| Anhydrous salt | Colour change on adding water |
|---|---|
| Cobalt(II) chloride | blue → pink |
| Copper(II) sulfate | white → blue |
Both tests only show that water is present - they say nothing about whether the water is pure.
Testing the Purity of Water
Pure water has a fixed melting point of 0°C and a fixed boiling point of 100°C (at standard atmospheric pressure). Dissolved impurities change these values:
- Impurities lower the melting point
- Impurities raise the boiling point
So measuring the melting point or boiling point of a water sample - and checking whether it matches exactly 0°C or 100°C - is how its purity is tested.
Worked Example
- Question: two test-tubes each contain a sample of water. One is pure; the other contains dissolved sodium chloride. Which test(s) can identify the pure sample - (1) measuring the boiling point of each, (2) testing each with acidified aqueous silver nitrate, (3) testing each with anhydrous copper(II) sulfate?
- Answer: 1 and 2. Pure water boils at exactly 100°C, while the salt solution boils above 100°C - test 1 distinguishes them. Silver nitrate gives a white precipitate (AgCl) with the Cl⁻ ions in the salt solution but no precipitate with pure water - test 2 also distinguishes them. Test 3 does not: anhydrous copper(II) sulfate turns blue in the presence of water regardless of purity, so both samples give the same result.
Distilled Water
Distilled water is used in practical chemistry (rather than tap water) because it contains far fewer dissolved chemical impurities. Tap water's dissolved ions and compounds could react with reagents or interfere with results, so distilled water is used to keep experiments and measurements accurate.
Common Mistakes
- ❌ Using the cobalt chloride or copper sulfate colour-change test to decide whether a water sample is pure
- ✓ those tests only detect the presence of water (any water, pure or not, turns them the same colour) - purity is tested separately, using melting point or boiling point
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