Coral reefs are built from calcium carbonate secreted by marine organisms. Scientists have observed that coral reefs are bleaching and dissolving in some regions of the ocean. Explain how human activities have led to conditions in the ocean that cause calcium carbonate structures to dissolve.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Coral reefs are among the most biodiverse ecosystems on Earth. They are constructed from calcium carbonate (CaCO₃) secreted by coral polyps and other marine organisms. In recent decades, scientists have recorded increasing rates of coral dissolution and bleaching across tropical oceans.
Model answer (5 marks)
Human activities such as burning fossil fuels and deforestation release large amounts of CO₂ into the atmosphere. The CO₂ is absorbed by the oceans, where it reacts with water to form carbonic acid (H₂CO₃). The formation of carbonic acid lowers the pH of seawater, making the ocean more acidic. In these more acidic conditions the calcium carbonate (CaCO₃) that makes up coral skeletons reacts with the acid and dissolves, breaking down the coral structures.
Examiner tips
- Mention the source of CO₂ (fossil fuels, deforestation).
- Show the chemical reaction CO₂ + H₂O → H₂CO₃.
- Explain the pH drop and its effect on CaCO₃ dissolution.
Common mistakes
- Confusing the source of CO₂ with the reaction in the ocean.
- Omitting the pH change or the acid–CaCO₃ reaction.
Mark scheme (5 marks)
- Human activities (burning fossil fuels / deforestation) release carbon dioxide (CO₂) into the atmosphere
- Carbon dioxide dissolves in seawater / is absorbed by the oceans
- Dissolved carbon dioxide reacts with water to form carbonic acid (H₂CO₃)
- This lowers the pH of the ocean / makes the ocean more acidic
- The more acidic conditions cause calcium carbonate to dissolve / react with the acid, breaking down coral structures
Key terms in this question
Related
- All OCR A-Level Chemistry B: Salters (H433) revision notes →
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- Decode the mark scheme abbreviations →
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