Chlorofluorocarbons (CFCs) were once widely used in aerosol sprays and refrigerators. Scientists discovered that CFCs cause damage to the ozone layer. Explain why the ozone layer is important and describe how CFCs cause damage to it.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The ozone layer sits in the stratosphere, approximately 15–35 km above the Earth's surface. International agreements such as the Montreal Protocol have restricted the use of CFCs since the late 1980s.
Model answer (5 marks)
The ozone layer absorbs harmful ultraviolet (UV) radiation from the Sun, protecting living organisms and ecosystems.
Without it, increased UV reaches the surface, raising skin cancer rates and damaging plants and marine life.
CFCs are released into the atmosphere and, in the stratosphere, are broken down by UV radiation, releasing chlorine atoms.
These chlorine atoms react with ozone (O₃), converting it into oxygen (O₂) and destroying the ozone molecule.
Because a single chlorine atom can catalyse the destruction of many ozone molecules, even small amounts of CFCs cause large‑scale ozone depletion.
Without it, increased UV reaches the surface, raising skin cancer rates and damaging plants and marine life.
CFCs are released into the atmosphere and, in the stratosphere, are broken down by UV radiation, releasing chlorine atoms.
These chlorine atoms react with ozone (O₃), converting it into oxygen (O₂) and destroying the ozone molecule.
Because a single chlorine atom can catalyse the destruction of many ozone molecules, even small amounts of CFCs cause large‑scale ozone depletion.
Examiner tips
- Use the exact terms ‘ozone layer’, ‘UV radiation’, ‘chlorine atoms’, and ‘catalyst’ to match the mark scheme.
- Show the chain of events: CFC → UV → chlorine → ozone → O₂, and note the catalytic role of chlorine.
- Keep each point brief but complete – one sentence per mark.
- Avoid unnecessary detail such as the Montreal Protocol unless asked.
Common mistakes
- Confusing UV radiation with visible light or not stating its harmful effects.
- Saying CFCs destroy ozone directly without mentioning the release of chlorine atoms.
- Claiming chlorine is permanently removed after one reaction instead of acting as a catalyst.
Mark scheme (5 marks)
- The ozone layer absorbs (harmful) ultraviolet (UV) radiation from the Sun
- Without the ozone layer, increased UV radiation reaching Earth's surface would cause harm to living organisms / increased skin cancer / damage to ecosystems
- CFCs are broken down / decomposed by UV radiation (in the stratosphere), releasing chlorine (free radicals / atoms)
- Chlorine (free radicals / atoms) react with ozone molecules, breaking them down / converting ozone (O₃) into oxygen (O₂)
- Chlorine acts as a catalyst / is regenerated / one chlorine atom can destroy many ozone molecules, so a small amount of CFC causes large-scale damage
Key terms in this question
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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