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.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
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.
Model answer (5 marks)
1. UV radiation splits O₂ into free O atoms.
2. Free O atoms combine with O₂ to form O₃.
3. O₃ absorbs UV, which splits it back into O₂ and O.
4. The ozone layer therefore filters harmful UV from reaching the surface.
5. If the layer thins, more UV reaches Earth, raising the risk of skin cancer, sunburn, DNA damage and ecosystem harm.
2. Free O atoms combine with O₂ to form O₃.
3. O₃ absorbs UV, which splits it back into O₂ and O.
4. The ozone layer therefore filters harmful UV from reaching the surface.
5. If the layer thins, more UV reaches Earth, raising the risk of skin cancer, sunburn, DNA damage and ecosystem harm.
Examiner tips
- Use the exact sequence of steps (UV → O → O₃ → UV → O₂) to show understanding. Include the protective role of the layer and the consequences of its loss. Keep each point brief and to the point to fit the 5‑mark limit.
Common mistakes
- Confusing the direction of the reactions (e.g., writing O₃ → O₂ + O as a formation step). Omitting the harmful effects of increased UV on living organisms. Using vague terms like "protects life" without specifying skin cancer or DNA damage.
Mark scheme (5 marks)
- UV radiation causes oxygen molecules (O₂) to split into individual oxygen atoms (free radicals / reactive oxygen atoms)
- These oxygen atoms react with oxygen molecules (O₂) to form ozone (O₃)
- Ozone (O₃) is also broken down naturally by absorbing UV radiation, splitting back into O₂ and an oxygen atom
- The ozone layer absorbs / filters out harmful UV radiation, preventing it from reaching Earth's surface
- 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 in this question
ozone · stratosphere · ultraviolet (UV) radiation · ozone layer
Related
- All OCR A-Level Chemistry B: Salters (H433) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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