# A student is researching the history of ozone depletion and the international response to it. Explain how the use of CFCs led to ozone depletion, and why an international agreement was needed to solve this problem.

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

> In 1987, the Montreal Protocol was signed by nations around the world. It aimed to phase out the production and use of ozone-depleting substances, including CFCs. Scientists had detected a significant thinning of the ozone layer, particularly over Antarctica, since the 1970s.

## Mark scheme (5 marks)

1. CFCs are broken down / decomposed by ultraviolet (UV) radiation in the upper atmosphere
2. This releases chlorine (free radicals / atoms) which react with / destroy ozone molecules
3. Chlorine acts as a catalyst / is not used up, so a single chlorine atom can destroy many ozone molecules
4. The ozone layer is in the upper atmosphere / stratosphere and ozone depletion increases the amount of UV radiation reaching Earth's surface, causing harm (e.g. increased skin cancer / damage to ecosystems)
5. An international agreement was needed because ozone depletion is a global problem — CFCs released in one country affect the ozone layer worldwide / no single country can solve it alone

## Key terms

- [CFCs](https://www.gradenine.co.uk/glossary/cfcs)
- [ozone depletion](https://www.gradenine.co.uk/glossary/ozone-depletion)

## 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-student-is-researching-the-history-8bc40c9e) · Published by Druglandscape Ltd.