# 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

> 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.

## 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

## 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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