Who this is for: Cambridge IGCSE Coordinated Science (0654) students who want water — the water cycle, tests, treatment and hardness — to become reliable marks instead of vague descriptions of “clean water”.
What query it owns: how to understand and revise water in Cambridge IGCSE Coordinated Science.
Why this is safe: this page owns the water revision-guide angle, while Tutopiya’s Water subtopic page owns the learning resource and the free Water quiz owns the practice.

Water is essential for life and a major topic in the Air and Water unit. Cambridge IGCSE Coordinated Science (0654) expects you to describe the water cycle, test for water with anhydrous copper sulfate or cobalt chloride, explain water treatment, distinguish hard and soft water, and state key chemical properties of water. This guide covers the syllabus content, comparison tables and the question types that appear every year.

Key takeaways

• The water cycle: evaporation → condensation → precipitation → collection; driven by solar energy.
• Test for water: anhydrous copper(II) sulfate turns blue (or anhydrous cobalt chloride paper turns pink).
• Test for purity: water boils at exactly 100 °C and freezes at exactly 0 °C at standard pressure (impure water has a range).
• Water treatment: filtration → sedimentation → chlorination (kills bacteria); optional fluoridation.
• Hard water contains dissolved calcium/magnesium ions; softened by ion exchange or adding sodium carbonate.

What is water in Cambridge IGCSE Coordinated Science?

The water topic covers the natural water cycle, chemical tests, treatment of domestic water supplies, hardness, and water’s role as a solvent and in reactions. Examiners test the water cycle stages, test observations, treatment steps and explain questions about hard water and limescale.

You can read the full explanation, diagrams and notes on Tutopiya’s Water subtopic page before you attempt questions.

The core ideas you must master

Idea	What it means	How the exam uses it
Water cycle	Continuous movement of water through states	”Describe the water cycle.”
Test for water	Anhydrous CuSO₄ → blue	”Describe a test for water.”
Water treatment	Make water safe to drink	”Describe how water is treated.”
Hard water	Ca²⁺/Mg²⁺ dissolved	”Explain why hard water forms limescale.”
Universal solvent	Dissolves many ionic/covalent substances	”State a chemical property of water.”

The water cycle — stages

Stage	Process	What happens
Evaporation	Liquid → gas	Water from oceans, lakes and rivers evaporates (energy from the Sun)
Transpiration	Plants release water vapour	Contributes to water vapour in the atmosphere
Condensation	Gas → liquid	Water vapour cools and forms clouds
Precipitation	Rain, snow, hail	Water returns to Earth’s surface
Collection	Run-off and groundwater	Water collects in rivers, lakes, oceans and aquifers
Repeat	Cycle continues	Driven by solar energy

Tests for water and purity

Test	Reagent	Result with water	Result without water
Water test	Anhydrous copper(II) sulfate (white)	Turns blue (hydrated CuSO₄·5H₂O)	Stays white
Water test	Cobalt chloride paper (blue)	Turns pink	Stays blue
Purity test	Measure boiling point	Exactly 100 °C (at 1 atm)	Impure: boils over a range, above 100 °C
Purity test	Measure freezing point	Exactly 0 °C (at 1 atm)	Impure: freezes below 0 °C

Hard water vs soft water

Feature	Hard water	Soft water
Dissolved ions	Ca²⁺ and/or Mg²⁺	Low levels of Ca²⁺/Mg²⁺
With soap	Forms scum (insoluble salts)	Lathers easily
Heating	Forms limescale (CaCO₃) in pipes/kettles	Minimal limescale
Health	Provides calcium (benefit)	—
Softening	Ion exchange; add sodium carbonate	Already soft

Water in past-paper wording: command words that matter

Command word / phrase	What the question wants	Typical water stem
Describe	Stages or observations	”Describe the water cycle.”
Explain	Cause and effect	”Explain why hard water does not lather easily with soap.”
State	Short factual answer	”State a chemical test for water.”
Give the equation	Balanced symbol equation	”Write the equation for the reaction of hard water with sodium carbonate.”
Suggest	Apply knowledge	”Suggest why water is chlorinated during treatment.”

Worked exam-style stems (how to answer the wording)

1. “Describe a chemical test for water.” Add a few drops of liquid to anhydrous copper(II) sulfate; if water is present, the white powder turns blue. Mark-scheme reward: anhydrous copper sulfate + turns blue.
2. “Explain why hard water forms limescale in kettles.” When hard water is heated, calcium hydrogencarbonate decomposes to form calcium carbonate (limescale), which deposits on surfaces. Reward: calcium/magnesium ions + heated + calcium carbonate/limescale forms.
3. “Describe how water is made safe to drink at a treatment works.” Water is filtered to remove solids, allowed to settle (sedimentation), then chlorinated to kill harmful bacteria. Reward: filtration + sedimentation + chlorination (any two valid steps).

Test yourself with the Water quiz.

How water connects to the rest of Coordinated Science chemistry

Water links to Air (composition of the atmosphere, combustion products) and underpins biology topics such as photosynthesis and transpiration. The Cambridge IGCSE Coordinated Science resource hub links every Chemistry subtopic.

Common mistakes students make

• Using hydrated copper sulfate to test for water (must be anhydrous — white).
• Confusing test for water with test for purity (different criteria).
• Stating hard water contains sodium ions (it contains calcium/magnesium ions).
• Forgetting chlorination kills bacteria during treatment.
• Describing evaporation without mentioning energy from the Sun.

When you need more support

If water questions keep costing marks, work through the Water quiz, then get focused help from a Cambridge IGCSE Coordinated Science tutor.

Frequently asked questions

How do you test for water? Anhydrous copper(II) sulfate turns from white to blue, or cobalt chloride paper turns from blue to pink.

What is hard water? Water containing dissolved calcium and/or magnesium ions that react with soap to form scum and deposit limescale when heated.

Why is water chlorinated? Chlorine kills harmful bacteria and microbes, making the water safe to drink.

How do I revise water effectively? Learn the water cycle, test observations, treatment steps and hard/soft water comparison, then take the Water quiz.

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