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?
- (a) Both Statement II and Statement III are correct and both of them explain Statement I
- (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 ✓ UPSC's answer
- (d) Neither Statement II nor Statement III is correct
Why the answer is (c)
• Porosity is the share of a rock's volume that is pore space; permeability is the ease with which water can flow through connected pores. The two are related but not the same.
• Statement II is correct and explains I: chalk is highly porous and its pores are interconnected, so it absorbs and transmits water freely — it is a classic aquifer rock.
• Statement III is wrong: clay is in fact very porous (often 40–50% pore space) but its pores are microscopic and poorly connected, so water cannot pass through — it is porous yet impermeable.
• So only one of II and III is correct, and that one (II) explains chalk's permeability in statement I.
• Hence option (c).
Why the other options are wrong
- (a) Both Statement II and Statement III are correct and both of them explain Statement I
- Statement III is wrong: clay is porous but impermeable.
- (b) Both Statement II and Statement III are correct but only one of them explains Statement I
- Statement III is wrong, so both cannot be correct.
- (d) Neither Statement II nor Statement III is correct
- Statement II is correct and explains chalk's permeability.
Asked in the GS Paper I of the UPSC Civil Services Preliminary Examination 2025, held on 25 May 2025. Question and answer key: Union Public Service Commission. Explanation: UPSC Answer Check.