Cooling Effect of Evaporation

What is evaporation?

Evaporation is the gradual change of state from liquid to gas that occurs at the surface of the liquid

Evaporation occurs when particles escape from a liquid. This happen even when the temperature is lower than the boiling point.

Factors affecting evaporation

i.   Temperature

    An increase of temperature increases the rate of evaporation

ii.  Surface area

    A larger surface area allows more particles to leave the liquid hence the larger the surface area the higher rate of evaporation

iii.  Concentration (Density)

     The concentration of a liquid that is evaporating in the surrounding air will determine the rate of its evaporation, if the surrounding air already has a high concentration, the rate of evaporation will be slow.  For example, clothes will dry faster in dry air but slowly in humid air

iv.    Rate of flow of air (wind)

In a windy environment, the evaporating particles are carried away as soon as they leave the surface of the liquid

Cooling Effect of Evaporation

When a liquid evaporates, it gives its latent heat of vaporization. If no heat is supplied from outside this results in cooling.

The human body uses evaporation to control body temperature. When it is too hot, sweat glands release water which then evaporates from the skin taking away latent heat of vaporization. This causes cooling in the body.

Cooling Effect of Evaporation in term of energies (Kinetic theory)

The liquid particles near the surface of liquid can escape and become gas particles (Evaporation).  The fastest particles (with the most kinetic energy) are most likely to evaporate from the liquid so the average speed and kinetic energy of the remaining particles decreases. This decrease in average energy means the temperature of the remaining particles falls then the liquid cool.