Which of the following statements is INCORRECT regarding water remaining colder in an earthen pot (matka)?

Correct — C. A matka cools by losing water, not by gaining it.

Water seeps through the pores of the unglazed clay, wets the outer surface and evaporates there. Evaporation needs latent heat, and it takes that heat from the pot and from the water still inside, so the temperature drops.

Option (c) reverses the direction of flow. Vapour moving inward and condensing would release heat rather than remove it, so as an account of the cooling it is the incorrect statement the question asks you to find.

Why the others are wrong

• (a)
  The earthen pot is porous. — True, so not the answer. Unglazed fired clay keeps a network of fine pores, and that porosity is the whole basis of the effect.
• (b)
  The water oozes out through the pores in an earthen pot. — True, so not the answer. Water oozing out through the pores is the first step: it is what keeps a wet film on the outside of the pot.
• (d)
  Water gets evaporated at the surface of the earthen pot. — True, so not the answer. Evaporation at the wet outer surface is where the cooling actually happens, because the latent heat it needs is drawn from the pot and its contents.

This is evaporative cooling, the same physics as sweating and as a desert cooler.

A liquid's fastest molecules escape first. What is left behind has less energy per molecule, so its temperature falls. The energy the escaping molecules carry off is the latent heat of vaporisation.

The pot only supplies the plumbing: pores carry water to a large outer surface where dry air can take it away. A glazed pot or a metal one cannot do this, because no water reaches the outside.

Key facts

• Evaporation is a cooling process because escaping molecules carry latent heat away from the liquid they leave.
• An earthen pot cools water because it is porous, letting water reach the outer surface and evaporate there.
• Evaporation is faster when the air is dry, hot and moving, and slower when humidity is high.
• A glazed or metal container does not cool this way, because water cannot reach its outer surface.

Common traps

• Picking the option that mentions pores, because the pot really is porous, when the question asks which statement is wrong
• Skimming past the word INCORRECT and marking the best true statement

No directly related past PYQ was found.