Authentic Classroom Explanation by IAS Monk

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📌 Question

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
(d) Neither Statement II nor Statement III is correct

✅ Correct Answer: (c)

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🎯 Theme of the Question

Physical Geography | Weathering | Groundwater | Porosity vs Permeability | Rock–Water Relationship

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🧠 Classroom Explanation

This is a classic UPSC concept trap where you must clearly distinguish between:

• Porosity → ability to hold water
• Permeability → ability to transmit water

Many aspirants confuse the two.

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Step 1: Understanding Statement I

Chalk is very permeable, clay is least permeable.

✔️ Correct.

• Chalk allows water to pass easily
• Clay strongly resists water movement

This is well-established in hydrology and geomorphology.

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Step 2: Testing Statement II

Chalk is porous and hence can absorb water.

✔️ Correct.
✔️ Also explains Statement I.

Why?

• Chalk is made of microscopic shell fragments
• This creates:
  • High porosity
  • Interconnected pore spaces
• Water not only enters chalk but also moves through it easily

➡️ Hence, chalk behaves as a good aquifer.

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Step 3: Testing Statement III

Clay is not at all porous.

❌ Incorrect.

This is the trick.

• Clay is porous, but:
  • Pores are extremely tiny
  • Pores are poorly connected
• Result:
  • Water may be held, but
  • It cannot flow freely

👉 Clay is porous but impermeable.

UPSC loves this exact contradiction.

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🔑 Core Concept Lock

Property	Chalk	Clay
Porosity	High	High (microscopic)
Permeability	High	Very low
Water movement	Easy	Difficult

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🧠 Why Option (c) is Correct

• Statement II ✔️ Correct and explains Statement I
• Statement III ❌ Incorrect
• Only one of II & III is correct and explains I

👉 Answer: (c)

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🧠 Memory Hook

“Chalk lets water walk, clay makes it wait.”

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🔍 Curiosity Corner

Why do clayey soils cause waterlogging?

Because:

• Water enters slowly
• Cannot drain downward
• Accumulates above clay layers

This is why:

• Clay-rich regions have poor natural drainage
• Chalk landscapes show springs and aquifers

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📘 Enrichment Note

UPSC Favourite Distinction:

• Porous ≠ Permeable
• A rock can:
  • Be porous but impermeable (clay)
  • Be both porous and permeable (chalk, sandstone)

This distinction appears repeatedly in:

• Groundwater questions
• Soil profiles
• Weathering & erosion

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🎯 Prelims Strategy Insight

Whenever UPSC mixes:

• “Porous”
• “Permeable”
• “Impermeable”

👉 Pause. Separate storage from movement.

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🧩 One-Line Ready Recall

Chalk: porous + permeable | Clay: porous but impermeable

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🧭 IAS Monk Whisper
Water remembers every pore, but moves only where paths connect.