Water

O Level / IGCSE Chemistry · Chemistry of the environment. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

This topic covers testing water, what natural water contains, and how water is made safe to drink.

A chemical test shows that water is present. It does not show that the water is pure. To check purity, measure the melting point and boiling point: pure water melts at exactly 0 °C and boils at exactly 100 °C. Distilled water is used in practical work because it contains fewer chemical impurities than tap water.

Water from rivers and lakes contains many substances. Some are useful, such as dissolved oxygen and some minerals. Others are harmful, such as toxic metal compounds, plastics, sewage, and nitrates and phosphates. The domestic water supply is treated in stages before it reaches homes.

Rules to remember

Common mistake

Using the cobalt(II) chloride or copper(II) sulfate test as proof that water is pure. These tests only show that water is present; purity is checked with the melting point or boiling point.

Worked example

A colourless liquid turns anhydrous copper(II) sulfate from white to blue. The liquid freezes at −2 °C. What can be concluded?

  1. The change from white to blue shows that the liquid contains water.
  2. Pure water freezes (and melts) at exactly 0 °C.
  3. −2 °C is below 0 °C, so the water has something dissolved in it.

Answer: The liquid contains water, but it is not pure water.

Practice questions

Try each one, then open the answer.

1. A sample of water boils at 101.5 °C at normal atmospheric pressure. What does this show?

  1. A
    it contains dissolved impurities
  2. B
    it has been distilled, so it is very pure
  3. C
    it is not water at all, but a different liquid
  4. D
    it is pure water, because water always boils at 101.5 °C
Show answer

Answer: A. Pure water boils at exactly 100 °C (and melts at 0 °C). Dissolved substances raise the boiling point.

2. Why can large amounts of nitrates and phosphates from fertilisers be harmful in rivers and lakes?

  1. A
    they make the water radioactive and so poisonous to fish
  2. B
    they cause deoxygenation, which harms aquatic life
  3. C
    they increase the dissolved oxygen, which makes fish grow too quickly
  4. D
    they kill all the microbes, so the water cannot be treated
Show answer

Answer: B. Nitrates and phosphates cause rapid growth of algae; when these die and decay, oxygen in the water is used up and fish can die.

3. In water treatment, why is chlorine added?

  1. A
    to remove solid particles
  2. B
    to kill bacteria
  3. C
    to soften the water
  4. D
    to improve the taste
Show answer

Answer: B. Chlorination sterilises the water. Filtration removes solids; sedimentation settles larger particles.

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