Explain why the oceans contain dissolved salts and describe how the concentration of these dissolved salts affects marine organisms that build calcium carbonate structures.

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).

The oceans cover over 70% of the Earth's surface and contain a vast store of dissolved substances. Scientists have noted that in some coastal regions, changes in the balance of dissolved substances are affecting marine life.

Model answer (5 marks)

Rocks on land are weathered and eroded, releasing minerals and salts that are carried by rivers into the oceans. The water evaporates, but the salts remain, so the concentration of dissolved salts in seawater increases over time. Marine organisms that build calcium carbonate structures, such as shells and skeletons, use dissolved calcium ions (Ca²⁺) and carbonate ions (CO₃²⁻) from seawater to precipitate CaCO₃. When the concentration of carbonate ions falls – for example, because of increased CO₂ dissolving and forming carbonic acid, which lowers pH – the organisms find it harder to form and maintain their calcium carbonate structures. This can lead to thinner, weaker shells or an inability to grow shells, threatening their survival.

Examiner tips

  • Show the weathering–river–ocean pathway first. Explain evaporation leaving salts behind. Describe Ca²⁺ + CO₃²⁻ → CaCO₃ formation. Link lower CO₃²⁻ to acidification and weaker shells. Use exact terms: carbonate ions, calcium carbonate, shells, acidity.

Common mistakes

  • Mixing up the roles of calcium and carbonate ions. Forgetting that evaporation concentrates salts. Using vague terms like "salts" without specifying calcium carbonate.

Mark scheme (5 marks)

  1. Rocks on land are weathered / eroded and minerals / salts are washed into the oceans by rivers
  2. Water evaporates from the oceans but salts remain, so salts become more concentrated over time
  3. Calcium carbonate structures (shells / skeletons) are built by marine organisms using dissolved calcium ions and carbonate ions from seawater
  4. If the concentration of carbonate ions decreases / seawater becomes more acidic, organisms struggle to form / maintain calcium carbonate structures
  5. This can result in thinner, weaker shells or inability to grow shells, threatening survival of these organisms

Key terms in this question

dissolved salts · calcium carbonate

Related

More Oceans questions

▶ Try answering this question with AI marking (free) →