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 ✓ UPSC's answer
(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

Why the answer is (a)

• Rainfall causes chemical weathering because rainwater is not pure: it carries dissolved gases from the atmosphere.

• Statement-II is correct: rainwater absorbs carbon dioxide to form weak carbonic acid, which dissolves limestone and alters feldspars (carbonation and hydrolysis) — an explanation of Statement-I.

• Statement-III is correct: rainwater also carries dissolved oxygen, which oxidises iron-bearing minerals, causing rusting and crumbling of rocks (oxidation) — another explanation of Statement-I.

• Rain also contributes to physical weathering through wetting-drying and by feeding frost action.

• Hence both II and III are correct and both explain I, option (a).

Why the other options are wrong

(b) Both Statement-II and Statement-III are correct, but only one of them explains Statement-I: Both dissolved CO2 and dissolved oxygen cause weathering, so both explain I.
(c) Only one of the Statements II and III is correct and that explains Statement-I: Both II and III are correct.
(d) Neither Statement-II nor Statement-III is correct: Both II and III are correct statements.

Asked in the GS Paper I of the UPSC Civil Services Preliminary Examination 2024, held on 16 June 2024. Question and answer key: Union Public Service Commission. Explanation: UPSC Answer Check.