Which of the following substances is mainly responsible for the depletion of the ozone layer?

Step 1: Analyze the question The question asks which substance is mainly responsible for destroying the ozone layer (the O₃ layer in the stratosphere that absorbs harmful UV radiation).

Step 2: Explain the concept Ozone (O₃) is constantly formed and broken down in the stratosphere. Certain man-made chemicals upset this balance. The main culprits are chlorofluorocarbons (CFCs), which were once widely used in refrigerators, air conditioners, aerosol sprays, and foam production.

How CFCs destroy ozone:

1. UV light breaks a CFC molecule and releases a chlorine atom: CCl₂F₂ → CClF₂• + Cl•
2. The chlorine atom attacks ozone: Cl• + O₃ → ClO• + O₂
3. The ClO reacts with free oxygen atoms and releases chlorine again: ClO• + O → Cl• + O₂
4. The freed chlorine atom attacks another ozone molecule. One chlorine atom can destroy thousands of ozone molecules.

Step 3: Evaluate the options

• A. Oxygen: Not harmful. O₂ is actually the raw material for forming ozone.
• B. Chlorofluorocarbon: The main cause of ozone depletion, as shown above.
• C. Carbon(II) oxide (CO): A toxic gas that binds to haemoglobin, but it is not the main ozone depleter.
• D. Nitrogen(II) oxide (NO): It can break down ozone in small amounts, but it is not the main cause. It is more associated with acid rain and smog.

Step 4: Answer The correct option is B. Chlorofluorocarbon.

Exam tip: Remember "CFC = Chlorine Frees Chaos". Whenever you see "ozone depletion" in an exam, think CFCs first. Also, don't confuse ozone depletion (CFCs) with global warming (CO₂ and CH₄) or acid rain (SO₂ and NOₓ). WASSCE often tests this distinction.