# A scientist is studying the chemistry of ozone in the stratosphere. Explain how ozone is both formed and broken down naturally in the stratosphere, and describe the role of ultraviolet radiation in these processes.

> OCR A-Level Chemistry B: Salters (H433) — OZ The ozone story · Explain · 5 marks

> Ozone (O₃) exists in a layer of the stratosphere where it absorbs harmful ultraviolet radiation. The concentration of ozone in this layer is maintained by a natural cycle of formation and breakdown reactions.

## Mark scheme (5 marks)

1. Oxygen molecules (O₂) absorb ultraviolet radiation (UV) and split into individual oxygen atoms / free oxygen atoms (O)
2. The free oxygen atom (O) reacts with an oxygen molecule (O₂) to form ozone (O₃)
3. Ozone (O₃) absorbs ultraviolet radiation and breaks down / is destroyed
4. The breakdown of ozone produces an oxygen molecule (O₂) and a free oxygen atom (O) / the oxygen atom can reform ozone
5. The formation and breakdown of ozone is a continuous / dynamic cycle that keeps ozone concentration roughly constant / ozone is constantly being made and destroyed naturally

## Key terms

- [ozone](https://www.gradenine.co.uk/glossary/ozone)
- [ultraviolet radiation](https://www.gradenine.co.uk/glossary/ultraviolet-radiation)
- [stratosphere](https://www.gradenine.co.uk/glossary/stratosphere)

## 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/a-scientist-is-studying-the-chemistry-e2f904f2) · Published by Druglandscape Ltd.