Describe the process of reverse osmosis and explain why this method of producing potable water is not widely used in the United Kingdom.

Eduqas GCSE Chemistry — C10 Using resources · Describe and Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Reverse osmosis is a pressure‑driven process in which water is forced through a semi‑permeable membrane. The membrane allows water molecules to pass but blocks dissolved salts, ions and other minerals, so the permeate is fresh, almost pure water.

The method is not widely used in the UK because it requires a large amount of energy to maintain the pressure needed for the membrane to work. Moreover, the UK already has abundant rainfall and surface water supplies, so the cost and energy demand of reverse osmosis are not justified for routine potable water production.

Examiner tips

  • Use the term ‘semi‑permeable membrane’ and mention ‘pressure‑driven’ to show understanding of the process.
  • Explain the energy requirement and link it to the UK’s ample freshwater resources to justify why the method is not common.
  • Keep the answer concise – 4 marks can be achieved with two short, clear points.
  • Use UK spelling (e.g., ‘programme’, ‘organisation’).

Common mistakes

  • Confusing reverse osmosis with distillation or saying the membrane removes only ‘salt’ instead of all dissolved ions.
  • Failing to mention the energy requirement or the UK’s sufficient rainfall as reasons for limited use.
  • Using vague phrases like ‘clean water’ without specifying the membrane’s role.

Mark scheme (4 marks)

  1. Reverse osmosis involves passing water through a partially permeable membrane (under pressure)
  2. The membrane removes dissolved salts/ions/minerals, producing fresh/pure water
  3. The process requires a large amount of energy
  4. The UK has sufficient rainfall / freshwater sources so there is no need for such an expensive/energy-demanding process

Key terms in this question

reverse osmosis · potable water

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