Evaporation

Evaporation is a process which a substance changes its state from liquid to gas without boiling. 

Cooling effect of evaporation

Evaporation cause cooling.

Reason:
Molecules in a liquid have different kinetic energies (different speeds). The more energetic molecules absorb heat from surrounding and overcome the intermolecular bonds between other molecules and escape into the atmosphere. The less energetic molecules are left behind. The average kinetic energy of the molecules in the liquid decreases, so temperature of liquid decreases.

Factors affecting evaporation

Boiling v.s Evaporation

Criteria Boiling Evaporation
Speed Quick process Slow process
Temperature at which it occurs Occurs at a fixed temperature Occurs at any temperature
Temperature change? Remains constant May change
Bubbles formed? Yes No
Takes place at…? Throughout the liquid Only at liquid surface
Energy supplied by…? External energy source Surroundings
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