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Learn0620Papers 1, 3 · CoreChemistry of the environmentWater
Chemistry of the environment · Subtopic 1 of 3

Water

Chemical tests show water is present: anhydrous copper(II) sulfate turns from white to blue; anhydrous cobalt(II) chloride from blue to pink. The physical test for pure water: it boils at exactly 100 ∘C100\,^\circ\mathrm{C} and melts at 0 ∘C0\,^\circ\mathrm{C}. Natural water contains useful substances (oxygen, minerals) and pollutants (sewage, nitrates, phosphates, metal compounds, plastics). Tap water is treated by sedimentation, filtration, chlorination (and sometimes carbon to remove tastes).

Cambridge 5070 Paper 1Cambridge 5070 Paper 2Cambridge 0620 Papers 2, 4Cambridge 0620 Papers 1, 3Edexcel 4CH1 Paper 1CEdexcel 4CH1 Paper 2C≈ 30 minRevision 1 · updated 2026-10-30
Section 1 of 14

Learning objectives

  • Describe chemical and physical tests for water.
  • Describe how drinking water is treated.
  • Identify beneficial and harmful substances in water.
Pause and think

Clear water from a mountain stream may still be unsafe to drink. Why?

Section 2 of 14

Core idea

Start hereSafe to drink?

About 2 billion people worldwide drink water contaminated with sewage. Clean tap water is one of the greatest life-savers in history, and chemistry makes it possible.

The big idea

Water is detected by colour-change tests and its purity shown by its boiling point. Drinking water is treated to remove solids and kill microbes.

1. Testing water

Worked exampleIs it water? Is it pure?

A colourless liquid turns white copper(II) sulfate blue, and boils at 101.5 ∘C101.5\,^\circ\mathrm{C}.

Goal
Conclusions.
Strategy
Separate the two tests.
  1. blue⇒water present\text{blue}\Rightarrow\text{water present}
    Chemical test.
  2. 101.5≠100⇒impure101.5\ne100\Rightarrow\text{impure}
    Dissolved substances raise the boiling point.
AnswerThe liquid contains water but is not pure (e.g. salt water).
✓ CheckSea water boils slightly above 100 °C ✓.

2. Treatment

Treating drinking water

Each stage removes something different.

Key takeawayPollutants Sewage (disease), nitrates and phosphates (eutrophication), metal compounds such as lead (toxic), plastics (harm wildlife).
Pause and think

Why might water from a well near farmland need testing for nitrates?

Summary

  • Tests: CuSO4\mathrm{CuSO_4} blue, CoCl2\mathrm{CoCl_2} pink.
  • Pure: 100 °C.
  • Sediment, filter, chlorinate.
Key words
Anhydrous
Without water of crystallisation.
Chlorination
Adding chlorine to kill microbes.
Eutrophication
Excess nutrients causing algal growth and oxygen loss in water.
HistoryJohn Snow. In 1854 London doctor John Snow traced a cholera outbreak to one contaminated water pump on Broad Street. Removing its handle helped end the outbreak and launched modern public health.
Fun factUniversal solvent. Water dissolves more substances than any other common liquid, which is why it's so hard to keep pure.
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