Evaporation:

Evaporation is defined as the “spontaneous escaping of molecules from a liquid surface at any temperature .”

• It is a surface phenomenon and takes place at all temperatures.
• All molecules in a liquid do not possess equal energies at any temperature so they move at different velocities. When high-energy molecules hit molecules on the surface of liquid they give part of their energy to surface molecules, which become able to overcome forces of attraction and thus leave the liquid surface. In this way evaporation takes place.
• As high-energy molecules escape from the liquid surface, therefore, the average kinetic energy of the remaining molecules decreases and hence temperature falls. It is for this reason that evaporation causes cooling.

e.g. water remains cool in a clay pitcher (Matka). The clay pitcher is a porous container from which water slowly comes out and evaporates due to which water in the pitcher gets cooled and due to continuous evaporation remains cool then.

Factors Affecting Evaporation:

Intermolecular forces:

The stronger the intermolecular forces less will be evaporation and vice versa. Strong forces firmly hold molecules together and thus liquid will evaporate slowly.

e.g. water is less volatile than ether and gasoline. Water has strong hydrogen bonding while the other two have weak London forces so the water evaporates slowly.

Rate of evaporation ∝ 1/Intermolecular forces

Temperature:

A higher temperature high will be the rate of evaporation and vice versa. At higher temperature energy of molecules will be high and they can easily overcome forces of attraction and will evaporate rapidly e.g. wet clothes soon get dried in summer than in winter.

Rate of evaporation ∝ Temperature 

Surface Area:

Evaporation is a surface phenomenon so it depends upon surface area, the more is the surface area more will the evaporation

and vice versa. e.g. water present in glass takes

more time to evaporate than if it is spread on the floor.

Wind:

If the wind is blowing evaporation rate will be fast.

• Impurities:

If non-volatile impurities are present, the rate of evaporation will be slow.

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