Two coloured powders tell you whether water is present, and two temperatures tell you whether it is pure. River water carries oxygen, minerals, sewage, plastics and fertiliser run-off, so it is settled, filtered, passed through carbon and chlorinated before it reaches a tap.
Water is detected with anhydrous copper(II) sulfate, which turns from white to blue, or anhydrous cobalt(II) chloride, which turns from blue to pink. These tests show water is present, not that it is pure: pure water boils at exactly 100 °C and melts at exactly 0 °C. Distilled water is used in practical chemistry because it contains no dissolved substances that could interfere with results. Natural water contains some beneficial substances (dissolved oxygen for aquatic life, some metal compounds as minerals) and some harmful ones (toxic metal compounds, plastics, sewage, harmful microbes, and nitrates and phosphates from fertilisers that lead to deoxygenation of water). Domestic water is treated by sedimentation and filtration to remove solids, carbon to remove tastes and odours, and chlorination to kill microbes.
A chemical test tells you water is there. A physical test tells you whether anything else is there too.
| Test | Result if water is present | What it shows |
|---|---|---|
| add to anhydrous copper(II) sulfate | white → blue | water is present |
| add to anhydrous cobalt(II) chloride (often on paper) | blue → pink | water is present |
| measure the boiling point | exactly 100 °C | the water is pure |
| measure the melting point | exactly 0 °C | the water is pure |
Sea water also turns copper(II) sulfate blue, because it contains water, but it boils above 100 °C because of the dissolved salt. Impurities raise the boiling point and lower the melting point. Distilled water is used in practical chemistry, for example to make up solutions, because tap water contains dissolved ions such as chloride and calcium that could react or give false test results.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Which colour change shows that a liquid contains water when it is added to anhydrous copper(II) sulfate?
What is seen when water is added to anhydrous cobalt(II) chloride?
Which stage of water treatment kills microbes?
Why is carbon used in water treatment?
Water from rivers, lakes and underground is never just H2O. Some of what it carries is useful; some is harmful.
| Substance | Source | Effect |
|---|---|---|
| dissolved oxygen | air, water plants | beneficial: needed by fish and other aquatic life |
| metal compounds | rocks, industry | some provide essential minerals (such as calcium); others are toxic (such as lead compounds) |
| plastics | litter, waste | harm aquatic life, which can swallow or be trapped by them |
| sewage | human and animal waste | contains harmful microbes that cause disease |
| nitrates and phosphates | fertiliser run-off from farmland | lead to deoxygenation of water, which kills aquatic life |
How nitrates and phosphates remove oxygen: they make algae grow rapidly; the algae block light and die; bacteria decompose the dead algae and use up the dissolved oxygen; fish and other animals suffocate. This chain is called eutrophication.
Water from a reservoir passes through the same stages in every treatment works. Learn them in order and with a reason for each.
Filtration removes insoluble solids only: dissolved salts, nitrates and microbes small enough to pass through are not removed by it. Chlorine kills microbes; it does not remove solids, tastes or dissolved substances. And a positive copper(II) sulfate test shows that water is present, never that it is pure.
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 2 and Paper 4 standard.
A colourless liquid turns anhydrous copper(II) sulfate blue and boils at 103 °C. What can be concluded?
Fertiliser washes into a lake and the fish die. Which explanation is correct?
Which substance in natural water is beneficial?
Why is distilled water, not tap water, used to make up solutions in practical chemistry?
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