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IGCSE · Practice questions · Water: tests, treatment and pollution

Test it, treat it.

Six original Cambridge-style questions on testing, purifying and polluting water.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Present, pure, polluted, treated.

01
Recall
[2 marks]

Describe a chemical test for water. Give the reagent and the colour change.

  • Add the liquid to anhydrous copper(II) sulfate. ✓
  • It turns from white to blue. (Or: anhydrous cobalt(II) chloride turns from blue to pink.) ✓
02
Explain
[3 marks]

A student says a sample of rain water is pure because it turns anhydrous copper(II) sulfate blue. Explain why the student is wrong and describe how to test whether the water is pure.

  • The colour change only shows that water is present; impure water gives the same result. ✓
  • Measure the boiling point (or melting point). ✓
  • Pure water boils at exactly 100 °C (and melts at exactly 0 °C). ✓
03
Recall
[4 marks]

Name the four stages in the treatment of the domestic water supply described in the syllabus and state the purpose of each.

  • Sedimentation: large insoluble particles settle out. ✓
  • Filtration: sand and gravel trap smaller insoluble particles. ✓
  • Carbon: removes tastes and odours. ✓
  • Chlorination: kills microbes. ✓
04
Explain
[4 marks]

Explain how nitrates from fertilisers washed into a river can cause fish to die.

  • Nitrates help algae and water plants grow rapidly. ✓
  • The algae die (they block light and run short of nutrients). ✓
  • Bacteria decompose the dead algae and use up dissolved oxygen. ✓
  • The water is deoxygenated, so fish cannot get enough oxygen and die. ✓
05
Recall
[3 marks]

Natural water contains substances that are beneficial and substances that are harmful. Give one beneficial and two harmful substances, with a reason for each.

  • Beneficial: dissolved oxygen, needed by aquatic life (or some metal compounds that provide essential minerals). ✓
  • Harmful: sewage, which contains harmful microbes that cause disease. ✓
  • Harmful: plastics, which harm aquatic life (or toxic metal compounds, or nitrates and phosphates that cause deoxygenation). ✓
06
Analysis
[2 marks]

Suggest why chlorination is the last stage of water treatment rather than the first.

  • Solids are removed first, so less chlorine is used up and microbes cannot shelter in particles. ✓
  • Adding chlorine last means some remains in the water as it travels to homes, keeping it free of microbes. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.