# 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.

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

> 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.

## Mark scheme (5 marks)

1. Human activities (burning fossil fuels / deforestation) release carbon dioxide (CO₂) into the atmosphere
2. Carbon dioxide dissolves in seawater / is absorbed by the oceans
3. Dissolved carbon dioxide reacts with water to form carbonic acid (H₂CO₃)
4. This lowers the pH of the ocean / makes the ocean more acidic
5. The more acidic conditions cause calcium carbonate to dissolve / react with the acid, breaking down coral structures

## Key terms

- [calcium carbonate](https://www.gradenine.co.uk/glossary/calcium-carbonate)

## Related

- [Revision notes for OCR A-Level Chemistry B: Salters (H433)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/coral-reefs-are-built-from-calcium-32b462f7) · Published by Druglandscape Ltd.