Explain why the pH of the world's oceans has been decreasing over the last 200 years, and describe the potential impact this change could have on marine organisms that build 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).

Since the Industrial Revolution, human activity has released large amounts of carbon dioxide into the atmosphere. Scientists have recorded a measurable decrease in ocean pH over this period, a process known as ocean acidification.

Model answer (5 marks)

Carbon dioxide released by human activity dissolves in seawater. The dissolved CO₂ reacts with water to form carbonic acid (H₂CO₃). This acid dissociates, releasing H⁺ ions and lowering the pH of the ocean.

The increased acidity reduces the saturation state of calcium carbonate (CaCO₃). As a result, CaCO₃ shells and skeletons become more soluble, causing them to dissolve or weaken. Organisms that build shells (e.g. molluscs, corals, some plankton) therefore have difficulty forming or maintaining their shells, which threatens their survival and can disrupt the marine food chain.

Examiner tips

  • Use the sequence: CO₂ input → dissolution → H₂CO₃ formation → H⁺ release → pH drop → CaCO₃ dissolution. Include the effect on shell‑building organisms and the food chain. Keep the answer concise and use the exact terminology (e.g. ‘saturation state’, ‘CaCO₃’).
  • Show the causal chain clearly; examiners reward linking the cause (CO₂) to the effect (shell dissolution).

Common mistakes

  • Confusing the role of CO₂ with other gases; forgetting that CO₂ forms carbonic acid. Failing to mention the impact on shell‑building organisms or the food chain. Using vague terms like ‘more acidic’ without explaining the chemical process.

Mark scheme (5 marks)

  1. Carbon dioxide from human activity (e.g. burning fossil fuels) dissolves in ocean water
  2. Dissolved carbon dioxide reacts with water to form carbonic acid
  3. Carbonic acid dissociates / ionises to release hydrogen ions, lowering the pH
  4. The lower pH / more acidic conditions cause calcium carbonate shells to dissolve or weaken
  5. Shell-building organisms struggle to form or maintain shells, threatening their survival / affecting the food chain

Key terms in this question

pH

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