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Why does evaporation cause cooling?

Class 9 ScienceNCERT chapter: Matter in Our Surroundingsइसे हिंदी में पढ़ें

Exam-ready answer

Evaporation causes cooling because the particles at the surface absorb heat energy from the surroundings to change into vapour. This heat is taken as latent heat of vaporisation, so the surroundings lose heat and become cooler. That is why water in an earthen pot or sweating cools us.

Explanation

Evaporation is the process in which a liquid changes into vapour at any temperature below its boiling point. It happens only at the surface of the liquid. The particles at the surface that have more kinetic energy escape into the air as vapour. To escape, they need energy, which they take from the liquid itself or from the surroundings.

The energy absorbed during evaporation is called latent heat of vaporisation. When these fast-moving particles leave, they carry away energy, so the average kinetic energy of the remaining particles decreases. As a result, the temperature of the liquid and its surroundings falls. This is why evaporation is a cooling process.

Everyday examples: When we sweat, the sweat evaporates by taking heat from our body, making us feel cool. Water kept in an earthen pot (matka) becomes cool because water seeps through tiny pores and evaporates, taking heat from the pot. Similarly, pouring water on a hot roof or floor cools it. In all these cases, evaporation causes cooling.

Points to remember

  1. Evaporation: liquid → vapour at any temperature below boiling point, only at the surface.
  2. Latent heat of vaporisation: heat energy absorbed by a liquid to change into vapour at constant temperature.
  3. Cooling occurs because the escaping high-energy particles take away heat from the surroundings.
  4. Rate of evaporation increases with temperature, surface area, wind speed, and decreases with humidity.
  5. Examples: sweating, earthen pot, water on hot roof, wet cloth in breeze.

Exam tip

Write the definition of evaporation, mention latent heat of vaporisation, and give at least one everyday example like sweating or earthen pot to score full marks.

Questions students also ask

Why does water in an earthen pot become cool?
Water seeps through the tiny pores of the earthen pot and evaporates. For evaporation, it absorbs latent heat from the remaining water and the pot, so the water inside becomes cool.
How does sweating cool our body?
When we sweat, the sweat evaporates by taking heat from our body. This loss of heat lowers our body temperature and gives a cooling sensation.
Does evaporation cause cooling in all liquids?
Yes, evaporation always causes cooling because the escaping particles absorb latent heat from the liquid and its surroundings, lowering the temperature.

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