Consider the following statements:Statement I: Rainfall is one of the reasons for weathering of rock

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Question

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?

Accepted Answer

Correct Answer: A — Both Statement II and Statement III are correct and both explain Statement IStep-by-step Solution:Statement I — Rainfall causes weathering of rocks: This is correct. Rainfall is a primary agent of chemical weathering of rocks through two key mechanisms — carbonation (via dissolved CO₂) and oxidation (via dissolved oxygen). ✓Statement II — Rainwater contains CO₂ in solution: This is correct. When rainwater falls through the atmosphere, it absorbs carbon dioxide (CO₂) from the air, forming a weak carbonic acid (H₂CO₃). This makes rainwater naturally slightly acidic (pH ~5.6). This carbonic acid is highly effective at dissolving limestone and carbonate rocks through a process called carbonation — producing limestone caves, karst topography, stalactites, and stalagmites. It also weathers feldspars in granite. ✓Statement III — Rainwater contains atmospheric oxygen: This is correct. Rainwater also dissolves atmospheric oxygen as it falls. This dissolved oxygen in rainwater causes oxidation of iron-bearing minerals in rocks — converting iron (Fe²⁺) to iron oxide/rust (Fe₃O₄, Fe₂O₃). This rusting weakens the rock structure. Many rocks have characteristic brownish-red colouration due to oxidation weathering. ✓Do both II and III explain I? Yes — rainfall causes weathering (Statement I) via two distinct chemical pathways: carbonation (because it contains dissolved CO₂ — Statement II) and oxidation (because it contains dissolved oxygen — Statement III). Both mechanisms independently and collectively explain how rainfall causes rock weathering.Why other options are incorrect:Option B: Incorrect. Both Statements II and III explain Statement I — not just one of them. Carbonation and oxidation are both valid and significant weathering mechanisms caused by rainwater.Option C: Incorrect. Both statements are correct — not just one.Option D: Incorrect. Both Statements II and III are factually correct.Key Concept: Chemical weathering processes: Carbonation: CO₂ + H₂O → H₂CO₃ (carbonic acid) → dissolves CaCO₃ (limestone) → karst topography; dissolves feldspars → clay minerals. Oxidation: O₂ + iron minerals → iron oxides (rust); weakens rock; produces reddish laterites. Hydration: Water combines with minerals to form hydrated compounds (e.g., anhydrite → gypsum); volume expansion causes mechanical stress. Hydrolysis: Water reacts with silicates (especially feldspars) to form clay minerals + silicic acid + metal hydroxides. Solution: Minerals dissolve in water (especially salts and limestone). Biological weathering: Organic acids from plant roots, lichens, and soil bacteria decompose rocks. Mechanical/Physical weathering: No change in chemical composition — includes frost action, thermal expansion/contraction, exfoliation, salt crystallisation, root wedging. India example: Deccan Plateau laterites — formed by intense chemical weathering of basalt under humid tropical conditions.

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?

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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?

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