UPSC Prelims 2024 Q7: Geography – Physical Geography & Geomorphology
Consider the following statements :
Statement - I : Rainfall is one of the reasons for weathering of rocks.
Statement - II : Rain water contains carbon dioxide in solution.
Statement - III : Rain water 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
Answer: (a) Both Statement - II and Statement - III are correct and both of them explain Statement - I
Explanation
Rain and chemical weathering of rocks
| Statement | Verdict | Explanation |
|---|---|---|
| I. Rainfall causes weathering | Correct | Water drives most chemical weathering processes |
| II. Rainwater contains dissolved CO₂ | Correct, explains I | CO₂ + H₂O forms weak carbonic acid, which dissolves limestone (carbonation) |
| III. Rainwater contains atmospheric oxygen | Correct, explains I | Dissolved O₂ attacks iron minerals, forming rust-like oxides (oxidation) |
Chemical weathering processes linked to rain:
| Process | Agent | Example |
|---|---|---|
| Carbonation | Carbonic acid | Limestone caves, karst |
| Oxidation | Dissolved oxygen | Reddish iron-rich rocks and soils |
| Hydration | Water absorbed into minerals | Swelling of clay minerals |
| Solution | Water dissolves soluble salts | Rock salt, gypsum |
Both II and III are true, and each separately explains how rain weathers rock.
Answer: (a) Both Statement - II and Statement - III are correct and both of them explain Statement - I.
Why each option is right or wrong
- (a) CORRECT. Dissolved CO₂ (carbonation) and dissolved O₂ (oxidation) each make rainwater a weathering agent.
- (b) Incorrect. Both II and III explain Statement I, not only one of them.
- (c) Incorrect. Both II and III are correct, not just one.
- (d) Incorrect. Rainwater does contain dissolved CO₂ and O₂; both statements are true.
Key point
Rain weathers rock chemically: dissolved CO₂ → carbonation; dissolved O₂ → oxidation.
Remember
Dissolved CO₂ and O₂ both make rainwater a weathering agent.
Common mistakes
- Thinking only CO₂ matters; oxidation by dissolved oxygen is equally important.
- Confusing chemical weathering (by rain) with physical weathering (frost, temperature change).
- Choosing 'only one explains I' without checking that each process acts on its own.
Exam tip
Rain = weak acid + oxygen. Carbonation dissolves limestone; oxidation rusts iron-bearing rocks.
Sources
- NCERT Class XI — Fundamentals of Physical Geography, Ch 6 (Geomorphic Processes — Weathering)
- Savindra Singh — Geomorphology (Chemical weathering)