Previous Year 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?

Both Statement II and Statement III are correct and both of them explain Statement I

Both Statement II and Statement III are correct, but only one of them explains Statement I

Only one of Statements II and III is correct and that explains Statement I

Neither Statement II nor Statement III is correct

Correct Answer: A

Detailed Solution

Statement I is correct: Rainwater is an important agent of chemical weathering. Water itself acts as a solvent, while the gases dissolved in it react chemically with minerals and disintegrate or weaken rocks.

Statement II is correct and explains Statement I:

Rainwater dissolves carbon dioxide while passing through the atmosphere.

Carbon dioxide reacts with water to produce weak carbonic acid:

CO₂ + H₂O ⇌ H₂CO₃

Carbonic acid particularly dissolves calcium carbonate present in limestone. This process is called carbonation.

It is important in the development of karst features such as solution holes, sinkholes and caves.

Statement III is correct and explains Statement I:

Atmospheric oxygen also dissolves in rainwater.

This oxygen reacts with minerals such as iron present in rocks and produces oxides.

This process is called oxidation. It may produce rust-like red, yellow or brown coatings on iron-bearing rocks and gradually weaken them.

Thus, dissolved carbon dioxide causes carbonation, while dissolved oxygen causes oxidation. Both processes explain how rainfall contributes to rock weathering.

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