A marine biologist is studying the shells of oysters in a coastal bay. She notices that newly formed shells are thinner and more fragile than shells collected 50 years ago. She suspects this is linked to changes in ocean chemistry caused by human activity. Explain why ocean chemistry has changed in this way and how this change affects the ability of oysters to form shells.

OCR A-Level Chemistry B: Salters (H433) — OC Oceans · Explain · 5 marks · View as Markdown

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

Oysters build their shells from calcium carbonate. The concentration of carbonate ions in seawater determines how easily marine organisms can extract the ions needed to build calcium carbonate shells.

Model answer (5 marks)

Increased burning of fossil fuels releases more CO₂ into the atmosphere. More CO₂ dissolves into the oceans, where it reacts with water to form carbonic acid, lowering the pH of seawater (ocean acidification). The more acidic water reacts with carbonate ions, dissolving them and reducing their concentration. With fewer carbonate ions available, oysters cannot obtain enough ions to build or maintain calcium carbonate shells, so the shells become thinner, weaker and harder to form.

Examiner tips

  • Use the exact sequence of cause–effect steps; each point is worth one mark.
  • Show the link between CO₂, pH, carbonate ions and shell formation.
  • Use the terminology ‘ocean acidification’, ‘carbonate ions’, ‘calcium carbonate shells’.
  • Keep the answer concise – 5‑point answer, no extra words.

Common mistakes

  • Confusing bicarbonate with carbonate ions.
  • Forgetting to mention the role of pH change (acidification).
  • Using vague terms like ‘more CO₂’ without linking to dissolution and acid formation.

Mark scheme (5 marks)

  1. Increased burning of fossil fuels has released more carbon dioxide into the atmosphere
  2. More carbon dioxide dissolves into the oceans
  3. Dissolved carbon dioxide reacts with water to form carbonic acid, which lowers the pH of the oceans (ocean acidification)
  4. The more acidic seawater reacts with / dissolves carbonate ions, reducing their concentration
  5. With fewer carbonate ions available, oysters cannot obtain enough ions to build / maintain calcium carbonate shells, so shells are thinner / weaker / harder to form

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