# 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

> 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.

## 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

- [dissolved salts](https://www.gradenine.co.uk/glossary/dissolved-salts)
- [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)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-the-oceans-contain-dissolved-0a0225bc) · Published by Druglandscape Ltd.