# Ultraviolet (UV) radiation from the Sun reaches Earth's atmosphere. The ozone layer plays a vital role in protecting life on Earth. Explain how ozone in the stratosphere is both formed and broken down naturally, and why damage to the ozone layer is harmful to living organisms.

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

> Scientists monitoring the atmosphere have recorded changes in the concentration of ozone in the stratosphere over several decades. Ozone is a molecule made of three oxygen atoms and is found mainly in the stratosphere.

## Mark scheme (5 marks)

1. UV radiation causes oxygen molecules (O₂) to split into individual oxygen atoms (free radicals / reactive oxygen atoms)
2. These oxygen atoms react with oxygen molecules (O₂) to form ozone (O₃)
3. Ozone (O₃) is also broken down naturally by absorbing UV radiation, splitting back into O₂ and an oxygen atom
4. The ozone layer absorbs / filters out harmful UV radiation, preventing it from reaching Earth's surface
5. If the ozone layer is damaged/thinned, more UV radiation reaches Earth, increasing the risk of harm to living organisms such as skin cancer / sunburn / damage to DNA / harm to ecosystems

## Key terms

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

## 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/ultraviolet-uv-radiation-from-the-sun-b2dc533f) · Published by Druglandscape Ltd.