Consider the following statements: Statement I: In the context of effect of water on rocks, chalk is known as a very permeable rock whereas clay is known as quite an impermeable or least permeable rock. Statement II: Chalk is porous and hence can absorb water. Statement III: Clay is not at all porous. Which one of the following is correct in respect of the above statements? (UPSC Prelims 2025)

Question

The correct answer is [C] Only one of the Statements II and III is correct and that explains Statement I. This question tests the specific distinction between porosity (the ability to hold water) and permeability (the ability to allow water to pass through).

Statement I (Correct): Chalk is a highly permeable rock because its structure allows water to flow through it easily. In contrast, clay is considered impermeable (or has very low permeability) because water cannot easily pass through its dense structure, despite its ability to hold water.
Statement II (Correct): Chalk is a porous rock (it has many tiny spaces or pores) and can indeed absorb water. Because these pores are interconnected, it also possesses high permeability, which directly explains why it acts as a permeable rock in Statement I.
Statement III (Incorrect): Clay is actually highly porous. It has a very high volume of pore spaces between its microscopic particles and can absorb a large amount of water (which is why it becomes sticky and expands). However, these pores are not interconnected, which prevents water from flowing through it. Therefore, clay is porous but impermeable.

Explanation of the Link: Statement II is correct and explains the first half of Statement I (why chalk is permeable). However, Statement III is scientifically false because clay is one of the most porous materials found in nature; its lack of permeability is due to the lack of connectivity between pores, not a lack of pores themselves. Since only Statement II is true among the secondary statements, it is the sole correct explanation for the behavior described in Statement I.

GKToday Admin · Accepted Answer

Only one of the Statements II and III is correct and that explains Statement I The correct answer is [C] Only one of the Statements II and III is correct and that explains Statement I. This question tests the specific distinction between porosity (the ability to hold water) and permeability (the ability to allow water to pass through).

Statement I (Correct): Chalk is a highly permeable rock because its structure allows water to flow through it easily. In contrast, clay is considered impermeable (or has very low permeability) because water cannot easily pass through its dense structure, despite its ability to hold water.
Statement II (Correct): Chalk is a porous rock (it has many tiny spaces or pores) and can indeed absorb water. Because these pores are interconnected, it also possesses high permeability, which directly explains why it acts as a permeable rock in Statement I.
Statement III (Incorrect): Clay is actually highly porous. It has a very high volume of pore spaces between its microscopic particles and can absorb a large amount of water (which is why it becomes sticky and expands). However, these pores are not interconnected, which prevents water from flowing through it. Therefore, clay is porous but impermeable.

Explanation of the Link: Statement II is correct and explains the first half of Statement I (why chalk is permeable). However, Statement III is scientifically false because clay is one of the most porous materials found in nature; its lack of permeability is due to the lack of connectivity between pores, not a lack of pores themselves. Since only Statement II is true among the secondary statements, it is the sole correct explanation for the behavior described in Statement I.

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