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UPSC Prelims 2025 GS Paper 1, question 28

Topic: World and physical geography

Q28

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?

  1. (a) Both Statement-II and Statement-III are correct and both of them explain Statement-I
  2. (b) Both Statement-II and Statement-III are correct but only one of them explains Statement-I
  3. (c) Only one of the Statements II and III is correct and that explains Statement-ICorrect answer
  4. (d) Neither Statement-II nor Statement-III is correct

Solution.

Answer: (c) Only one of the Statements II and III is correct and that explains Statement-I

  1. Statement-I is correct: chalk is a permeable rock that allows water to pass through its pore spaces and joints, whereas clay is practically impermeable.
  2. Statement-II is correct and explains Statement-I: chalk is a porous limestone rock capable of absorbing and transmitting water.
  3. Statement-III is incorrect: clay is actually highly porous (often holding up to 50% water by volume in microscopic pores), but it is impermeable because its pore spaces are extremely tiny and poorly interconnected, trapping water by surface tension.

Trap: Porosity and permeability are distinct: clay has high porosity but very low permeability.