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10.1Chemistry of the Environment

Water and Water Pollution — Cambridge O Level Chemistry 5070 Revision Notes & Topical Questions

5 min read 13 parts Lesson 1 of 3

📚 Cambridge O Level Chemistry 5070 Hub / Topical Revision
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Lesson summary

Describe tests for water and its purity, substances in natural water, and the stages of water treatment.

Part 1 of 11Chemical tests for water

  • Cobalt(II) chloride
    • blue anhydrous CoCl2 turns pink with water.
  • Equation
    • CoCl2(s) + 6H2O(l) ⇌ CoCl2·6H2O(s).
    • Form
      • often used as cobalt chloride paper.
  • Copper(II) sulfate
    • white anhydrous CuSO4 turns blue with water.
  • Equation
    • CuSO4(s) + 5H2O(l) ⇌ CuSO4·5H2O(s).
    • Note
      • these tests show water is present, not that it is pure.

Part 2 of 11Purity of water

  • Pure substances
    • melt and boil at sharp, specific temperatures.
  • Water
    • boils at 100 °C and melts at 0 °C.
  • Mixtures
    • melt and boil over a range of temperatures.
  • Impurities in water
    • raise the boiling point and lower the melting point.
    • Use
      • melting and boiling data show whether water is pure.
  • Distilled water
    • water that is vaporised and condensed again, containing very few impurities.
  • Practical use
    • used in chemistry because tap water has impurities that can interfere with reactions.

Part 3 of 11Uses and natural sources of water

  • Domestic
    • drinking, cooking, gardening and sanitation.
  • Agricultural
    • drink for animals and watering crops.
  • Industrial
    • solvent, coolant and steam for generating electricity.
  • Sources
    • lakes, rivers and groundwater.
    • Aquifer
      • a rock that stores water.

Part 4 of 11Substances in natural water

  • List
    • dissolved oxygen, metal compounds, plastics, sewage, harmful microbes, nitrates from fertilisers and phosphates from fertilisers and detergents.
  • Entry
    • rain washes many of these into lakes, rivers and groundwater.
    • Origin
      • some are natural but many come from human activity.
    • Caution
      • natural water is not automatically clean.

Part 5 of 11Beneficial and harmful substances

SubstanceBeneficial effectHarmful effect
Dissolved oxygenEssential for aquatic life—
Metal compoundsCalcium and magnesium provide essential mineralsLead, arsenic, mercury and aluminium compounds are toxic
Plastics—Trap or starve aquatic animals
Sewage—Contains microbes that cause disease
Nitrates and phosphates—Promote plant growth that removes oxygen, causing eutrophication

Part 6 of 11Stages of water treatment

  1. Water is taken from rivers, reservoirs or groundwater.
  2. Sedimentationwater stands in tanks and mud and sand sink to the bottom.
  3. Filtrationlayers of sand and gravel trap smaller particles.
  4. Carbon (charcoal) filterremoves tastes and odours.
  5. Chlorinationchlorine is added to kill bacteria and microorganisms.
  • Why chlorine
    • microbes are too small for filters.
  • Diseases prevented
    • cholera and typhoid.
    • Impurities
      • insoluble ones include soil and plant matter, soluble ones include dissolved calcium and metal compounds.
    • Exam focus
      • state the purpose of each stage.

Part 7 of 11Examples of exam reasoning

  • Test for water
    • use the colour change of anhydrous copper(II) sulfate from white to blue or cobalt chloride paper from blue to pink.
  • Purity
    • a pure sample has a sharp fixed boiling point, so a range shows a mixture.
    • Distillation
      • the method for making distilled water.
  • Treatment purposes
    • link each stage to its job.
  • Sedimentation
    • removes large solids.
    • Filtration
      • removes small solids.
    • Charcoal
      • removes taste and smell.
    • Chlorine
      • kills microbes.

Part 8 of 11Water pollution sources

SourcePollutantProblem
FertilisersNitrates and phosphatesExcess plant growth and loss of oxygen
DetergentsPhosphatesExcess plant growth
Industrial wasteMetal compoundsToxic to living things
SewageMicrobesDisease such as cholera and typhoid
LitterPlasticsHarm to aquatic animals

Part 9 of 11Key facts to recall

  • Water tests
    • cobalt chloride blue to pink and copper sulfate white to blue.
  • Pure water
    • boils at exactly 100 °C and melts at 0 °C.
    • Sedimentation then filtration then chlorination
      • the usual order of treatment.

Part 10 of 11Why water must be treated

  • Safety
    • untreated water can carry microbes that cause disease.
  • Taste
    • some dissolved substances and organic matter give an unpleasant taste or smell.
    • Result
      • the stages remove solids, tastes and microbes in turn.

Part 11 of 11Worked examples

  1. Describe a chemical test for water. [2]2 marks

  2. A liquid boils at 102 °C. What does this show? [2]2 marks

  3. State the purpose of chlorination. [1]1 mark

  4. Explain why fertiliser run-off harms aquatic life. [2]2 marks

Key terms

TermMeaning
AnhydrousContaining no water of crystallisation.
Distilled waterWater that has been vaporised and condensed, with few impurities.
AquiferA rock that stores water underground.
SedimentationParticles settling to the bottom of a liquid under gravity.
ChlorinationAdding chlorine to kill microorganisms.
EutrophicationDamage to water bodies caused by excess nutrients leading to oxygen loss.

Avoid these mistakes

  1. Saying the cobalt chloride test proves the water is pure — it only shows water is present.
  2. Saying filtration removes bacteria — chlorine does that.
  3. Saying all metal compounds in water are harmful — some are essential.
  4. Saying impurities lower the boiling point — they raise it.

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