Authentic Classroom Explanation by IAS Monk

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📍 The Question

Consider the following statements:

Statement-I:
Rainfall is one of the reasons for weathering of rocks.

Statement-II:
Rainwater contains carbon dioxide in solution.

Statement-III:
Rainwater contains atmospheric oxygen.

Which one of the following is correct in respect of the above statements?

(a) Both Statement-II and Statement-III are correct and both of them explain Statement-I
(b) Both Statement-II and Statement-III are correct, but only one of them explains Statement-I
(c) Only one of the Statements II and III is correct and that explains Statement-I
(d) Neither Statement-II nor Statement-III is correct

Correct Answer: (a) Both Statement-II and Statement-III are correct and both of them explain Statement-I

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🎯 Theme of the Question

Physical Geography | Geomorphology | Chemical Weathering | Denudation Processes

This question tests conceptual understanding of chemical weathering, not mere definition-based knowledge.

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🧠 Classroom Explanation

Weathering refers to the in-situ decomposition of rocks due to exposure to air, water, and biological agents.
Among these, chemical weathering is the most fundamental process.

Let us analyse how rainfall contributes.

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🔹 Statement-I: Rainfall is one of the reasons for weathering of rocks. ✅

• Rainfall supplies:
  • Water
  • Dissolved gases
  • Chemical agents
• These initiate chemical reactions that weaken rocks

✔️ Statement-I is correct

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🔹 Statement-II: Rainwater contains carbon dioxide in solution. ✅

• Atmospheric CO₂ dissolves in rainwater
• This forms carbonic acid (a weak acid)
• Leads to solution weathering, especially in:
  • Limestone (calcium carbonate)
• Widens joints, cracks, caves, and passages

✔️ Statement-II is correct
✔️ It explains Statement-I

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🔹 Statement-III: Rainwater contains atmospheric oxygen. ✅

• Rainwater absorbs oxygen from the atmosphere
• This oxygen causes oxidation weathering
• Common example:
  • Iron minerals converting to iron oxide (rust)
• Oxidation weakens rock structure, making it easier to erode

✔️ Statement-III is correct
✔️ It also explains Statement-I

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📊 Final Assessment

Statement	Status	Explains Statement-I
Rainfall causes weathering	✅	—
CO₂ in rainwater	✅	✅
Oxygen in rainwater	✅	✅

👉 Both Statements II and III are correct and both explain Statement-I

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❌ UPSC Elimination Logic

• UPSC checks whether aspirants:
  • Understand mechanisms, not just facts
• Rainfall acts as:
  • A carrier of chemical agents
  • Not merely as mechanical water

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🧩 Memory Hook

“Rain weathers rocks not by force, but by chemistry.”

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🧠 Prelims Strategy Insight

For weathering questions:

• Always think in terms of processes:
  • Solution (carbonic acid)
  • Oxidation (oxygen)
  • Organic acids
• Rainfall is the delivery system for all three

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🧭 IAS Monk Whisper

Rocks crumble not when struck, but when slowly persuaded by water.