Correct Option
Statement-I is correct. Rainfall is a significant agent of weathering, contributing to both physical and chemical breakdown of rocks. Its role in chemical weathering is particularly notable through the interaction of dissolved atmospheric gases with rock minerals.
Statement-II is correct. Rainwater absorbs carbon dioxide (CO₂) from the atmosphere as it falls. This dissolved CO₂ reacts with water to form carbonic acid (H₂CO₃), a weak acid. This carbonic acid then reacts with various rock minerals, especially calcium carbonate in calcareous rocks like limestone, leading to their dissolution and contributing to chemical weathering. Thus, Statement-II provides a valid explanation for Statement-I.
Statement-III is correct. Atmospheric oxygen (O₂) also dissolves in rainwater. This dissolved oxygen facilitates oxidation reactions with certain minerals present in rocks, particularly those containing iron. For instance, ferrous iron (Fe²⁺) in minerals can be oxidized to ferric iron (Fe₂O₃), commonly known as rusting. This oxidation process weakens the rock structure and contributes to chemical weathering. Therefore, Statement-III also provides a valid explanation for Statement-I.
Since both Statement-II and Statement-III are factually correct and both describe distinct mechanisms through which rainfall causes chemical weathering of rocks, they both explain Statement-I.
Incorrect Options
Option 2 is incorrect because both Statement-II and Statement-III explain Statement-I, not just one of them.
Option 3 is incorrect because both Statement-II and Statement-III are correct, not just one of them.
Option 4 is incorrect because both Statement-II and Statement-III are factually correct statements.