Water
Testing for water, water treatment, and the uses of water in industry and the home.
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 10.1.1Describe chemical tests for the presence of water and for its purity.
- 10.1.2Describe the treatment of the domestic water supply.
5 minute read
Water
Water looks like the simplest substance in chemistry, which is exactly why exam questions on it focus on two things: how do you prove a liquid is water, and how do you prove it is pure.
Testing for water
Two chemical tests are used, and neither on its own proves purity — they only prove the liquid contains water:
- Anhydrous copper(II) sulfate is white. Add a drop of the liquid: if it turns blue, water is present (it rehydrates to hydrated copper(II) sulfate, CuSO₄·5H₂O).
- Anhydrous cobalt(II) chloride paper is blue. Add a drop of the liquid: if it turns pink, water is present.
Testing for purity
Colour-change tests only show water is present, not that it is pure. Purity is a physical test: pure water has a sharp, fixed boiling point of exactly 100 °C and a sharp, fixed melting/freezing point of exactly 0 °C, both at normal atmospheric pressure. Dissolved substances raise the boiling point and lower the melting point, and they turn a sharp melting/boiling point into a range — so measuring the boiling or melting point is the definitive purity test.
Treating the domestic water supply
Water from rivers or reservoirs is treated in stages before it reaches taps: screening removes large debris, sedimentation (often helped by adding a flocculant that makes fine particles clump and settle) removes suspended solids, filtration through sand removes remaining fine particles, and chlorination kills harmful microorganisms. Sometimes fluoride is also added to help protect teeth.
Think of it like this
The colour-change tests are like a smoke alarm: they tell you water is present, but not whether the air is otherwise clean. Only measuring the boiling/melting point tells you the whole sample is trustworthy.
Worked examples
Method, step by step
A student has a colourless liquid. Cobalt(II) chloride paper turns pink when dipped in it, and it boils at 100 °C exactly. What can be concluded?
- 1Pink cobalt chloride paper confirms water is present.
- 2A sharp boiling point of exactly 100 °C at atmospheric pressure confirms the water is pure (no dissolved solutes).
The liquid is pure water.
Common misconceptions
- Thinking a positive cobalt chloride or copper sulfate colour change proves the water is pure — it only proves water is present; a dissolved-salt solution gives the same colour change.
- Believing distilled water and "pure" water are the same as tap water — tap water is treated for safety, not purity, and still contains dissolved minerals and chlorine.
In the exam
- If asked "how would you show this liquid is pure water", the answer must be a physical test (boiling or melting point), not the colour-change chemical tests.
- Give both directions for impurities: they raise the boiling point AND lower the melting point compared with the pure substance.