Q1. [1] § 13.2.1 Ozone Layer and How it is Getting Depleted
Assertion (A) : Oxygen is essential for all aerobic forms of life.
Reason (R) : Free oxygen atoms combine with molecular oxygen to form ozone.
- A Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of Assertion (A).
- B Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of Assertion (A).
- C Assertion (A) is true, but Reason (R) is false.
- D Assertion (A) is false, but Reason (R) is true.
Previously asked in CBSE board exam
2024 31/4/1 Q20
Generated by claude-sonnet-4-6 · 2026-06-15 06:42 · grounding rag
Model Answer
Answer: B
Both A and R are true, but R is not the correct explanation of A. Oxygen (O₂) is essential for aerobic life, and free oxygen atoms do combine with O₂ to form ozone — but ozone formation does not explain why O₂ is essential for life.
Explanation
- Assertion is true: O₂ is essential for all aerobic forms of life (used in respiration).
- Reason is also true: UV radiation splits O₂ → free O atoms, which then combine with O₂ to form O₃ (ozone) — this reaction is correctly described.
- However, ozone formation is not the reason O₂ is essential for life; O₂ is needed for cellular respiration, which is a completely different concept. So R does not explain A → Option B.
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