Q7 (IAS/2024) Geography › World Physical Geography › Weathering and erosion Official Key

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 the Statements II and III is correct and that explains Statement-I

Neither Statement-II nor Statement-III is correct

Result

Your answer: — · Correct: A

Explanation

The correct answer is option A because both Statement-II and Statement-III are factually correct and both explain how rainfall causes weathering of rocks.

Rainfall is naturally acidic with a pH of ~5.6 as CO2 dissolves in the rainwater producing weak carbonic acid[1]. Rain-water contains enough carbon dioxide to make it a weak acid, and solution is the most potent weathering process in limestone regions because the rain-water attacks and dissolves the calcium carbonate[2]. This confirms Statement-II is correct and explains Statement-I.

Chemical weathering processes including solution, carbonation, hydration, oxidation and reduction act on rocks, and water and air (oxygen and carbon dioxide) along with heat must be present to speed up all chemical reactions[3]. Oxidation is the reaction of oxygen in air or water with minerals in the rock, for example, iron in rocks when in contact with air is changed to iron oxide[4]. This confirms Statement-III is correct and also explains Statement-I.

Since both statements are correct and both independently explain how rainfall causes weathering (through carbonation and oxidation respectively), option A is the correct answer.

Sources

[1] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 6: Geomorphic Movements > Anthropogenic Solution Weathering > p. 91
[2] Certificate Physical and Human Geography , GC Leong (Oxford University press 3rd ed.) > Chapter 4: Weathering, Mass Movement and Groundwater > CHEMICAL WEATHERING > p. 36
[3] FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.) > Chapter 5: Geomorphic Processes > Chemical Weathering Processes > p. 40
[4] Certificate Physical and Human Geography , GC Leong (Oxford University press 3rd ed.) > Chapter 4: Weathering, Mass Movement and Groundwater > CHEMICAL WEATHERING > p. 37

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Don’t just practise – reverse-engineer the question. This panel shows where this PYQ came from (books / web), how the examiner broke it into hidden statements, and which nearby micro-concepts you were supposed to learn from it. Treat it like an autopsy of the question: what might have triggered it, which exact lines in the book matter, and what linked ideas you should carry forward to future questions.

Q. Consider the following statements : Statement-I : Rainfall is one of the reasons for weathering of rocks. Statement-II : Rain water c…

At a glance

Origin: From standard books Fairness: High fairness Books / CA: 10/10 · 0/10

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A classic 'Sitter' derived straight from the fundamentals of Chemical Weathering. It rewards aspirants who didn't just memorize landforms but understood the chemical equations (Oxidation and Carbonation) behind them. If you read NCERT Class XI Geomorphic Processes or GC Leong Ch 4, this was unmissable.

How this question is built

This question can be broken into the following sub-statements. Tap a statement sentence to jump into its detailed analysis.

Statement 1: Is rainfall a cause of the weathering of rocks?
Statement 2: Does rainwater contain dissolved carbon dioxide (CO2) from the atmosphere?
Statement 3: Does dissolved carbon dioxide in rainwater form carbonic acid that chemically weathers rocks?
Statement 4: Does rainwater contain dissolved atmospheric oxygen (O2)?
Statement 5: Does dissolved atmospheric oxygen in rainwater contribute to the weathering of rocks through oxidation?

Statement 1

Is rainfall a cause of the weathering of rocks?

Origin: Direct from books Fairness: Straightforward Book-answerable

From standard books

Certificate Physical and Human Geography , GC Leong (Oxford University press 3rd ed.) > Chapter 4: Weathering, Mass Movement and Groundwater > CHEMICAL WEATHERING > p. 36

Presence: 5/5

“But much of the weathered material, or regolith (remains of the rock), may stay in position forming the basis of soil. Regolith is simply the mineral remains of decomposed rocks, but soil contains organic materials, such as the roots of plants; fallen leaves, small animals such as worms, bacteria and so on. When a soil cover exists, chemical weathering of the underlying rocks does not cease; on the contrary it is usually enhanced. This is because the soil absorbs rain-water and keeps the underlying rocks in contact with this moisture. The rain-water absorbs organic acids from the soil and thus becomes a stronger weathering agent than pure rain-water acting on bare rock.”

Why this source?

States that soil absorbs rain-water and keeps underlying rock moist, enhancing chemical weathering
Notes rain-water picks up organic acids in soil and becomes a stronger agent of rock decomposition

FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.) > Chapter 5: Geomorphic Processes > Physical Weathering Processes > p. 41

Presence: 4/5

“forces could be: (i) gravitational forces such as overburden pressure, load and shearing stress; (ii) expansion forces due to temperature changes, crystal growth or animal activity; (iii) water pressures controlled by wetting and drying cycles. Many of these forces are applied both at the surface and within different earth materials leading to rock fracture. Most of the physical weathering processes are caused by thermal expansion and pressure release. These processes are small and slow but can cause great damage to the rocks because of continued fatigue the rocks suffer due to repetition of contraction and expansion.”

Why this source?

Identifies water pressures from wetting and drying cycles as a force that fractures rock
Links repeated moisture-driven expansion/contraction to physical weathering and rock fatigue

Science-Class VII . NCERT(Revised ed 2025) > Chapter 5: Changes Around Us: Physical and Chemical > 5.7.1 Weathering of rocks > p. 67

Presence: 4/5

“The red colour is a result of the iron oxide produced when the rock surface was exposed for a long time to water or air containing water vapour. These physical and chemical changes in rocks are collectively called weathering, which eventually leads to the formation of soil.”

Why this source?

Describes exposure to water or moist air producing chemical changes (e.g., iron oxide) in rock
Classifies those physical and chemical changes caused by water exposure collectively as weathering

Statement analysis

This statement analysis shows book citations, web sources and indirect clues. The first statement (S1) is open for preview.

Statement analysis

This statement analysis shows book citations, web sources and indirect clues. The first statement (S1) is open for preview.

Statement analysis

This statement analysis shows book citations, web sources and indirect clues. The first statement (S1) is open for preview.

Statement analysis

This statement analysis shows book citations, web sources and indirect clues. The first statement (S1) is open for preview.

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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 ?

Explanation

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SIMILAR QUESTIONS

Statement I: Chemical weathering processes are found more active in hot and humid environment. Statement II: High temperature and rainfall help in the process of decomposition of rocks.

Statement I : The process of decay and disintegration of rocks in situ is called weathering. Statement I : Weathering takes place both mechanically and chemically.

Consider the following statements : Statement-I : The temperature contrast between continents and oceans is greater during summer than in winter. Statement-II : The specific heat of water is more than that of land surface. Which one of the following is correct in respect of the above statements?