# Explain why the pH of the world's oceans has been decreasing over the last 200 years, and describe the potential impact this change could have on marine organisms that build shells.

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

> Since the Industrial Revolution, human activity has released large amounts of carbon dioxide into the atmosphere. Scientists have recorded a measurable decrease in ocean pH over this period, a process known as ocean acidification.

## Mark scheme (5 marks)

1. Carbon dioxide from human activity (e.g. burning fossil fuels) dissolves in ocean water
2. Dissolved carbon dioxide reacts with water to form carbonic acid
3. Carbonic acid dissociates / ionises to release hydrogen ions, lowering the pH
4. The lower pH / more acidic conditions cause calcium carbonate shells to dissolve or weaken
5. Shell-building organisms struggle to form or maintain shells, threatening their survival / affecting the food chain

## Key terms

- [pH](https://www.gradenine.co.uk/glossary/ph)

## 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)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-the-ph-of-the-bebf22a3) · Published by Druglandscape Ltd.