Explain why water is described as a 'universal solvent' and state one problem that arises from this property when water passes through soil and rock.

Cambridge International IGCSE Chemistry (0620) — 10.1 Water · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Water is a polar molecule, so its molecules have a partial negative charge near the oxygen and a partial positive charge near the hydrogens. This polarity allows water to attract and surround a wide range of solutes, from ionic salts to polar molecules, and therefore it dissolves many substances.

Because it dissolves minerals and salts from soil and rock, the water becomes hard, containing dissolved calcium and magnesium ions.

Hard water does not lather easily with soap and can leave a scum or limescale in pipes and kettles.

Examiner tips

  • Use the word ‘polar’ and explain the attraction to solutes; mention minerals/salts to show breadth of solubility; give a clear example of the problem (hard water).
  • Show the link between polarity, dissolution, and the resulting hardness.

Mark scheme (4 marks)

  1. Water dissolves a very large number / wide range of substances / solutes
  2. This is because water is a polar molecule / water molecules are polar, allowing it to interact with / attract many solutes
  3. Water dissolves minerals / salts / ions from soil and rock, so the water becomes hard / contains dissolved calcium or magnesium ions / compounds
  4. Hard water does not lather easily with soap / causes scum with soap OR causes scale / limescale to form in pipes and kettles

Key terms in this question

universal solvent

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