Evaporation?

Question

Consider a drop of water on a table. U cant find it after some time. Because it evaporates. But for water to evaporate at a pressure of 1 bar its temperature should increase to 100 degree centigrade. Then why did the water evaporate

Answer 1

The partial pressure of water doesn't need to reach 1 bar (1 atm) to evaporate. If the partial pressure of water, in the surrounding air is less than the partial pressure the water exerts, at that temperature, then the water continues to evaporate.

For the water to fail to evaporate, it would mean that the air was saturated (the partial pressure exerted by the gaseous water was equal to or greater than the partial pressure water would have at that temperature).

Answer 2

OK this isn't too hard. Liquid exert their vapor pressure on the surroundings. At 100 C the vapor pressure of water is equal to one atmosphere so it can displace all of the air and occupy 100% of its surroundings.

However at for example 72 F the vapor pressure of water is only 0.39 psia. Or about 0.03 bar. Which means water can displace about 3% of the air by volume. So if the air contains less than 3% by volume water will evaporate. The water still has to absorb around 1000 BTU/lb in order to evaporate so if the water that evaporates absorbs heat from the surrounding water the temperature of the rest of the water could drop below the ambient temperature.

Relative humidity is a measure of how much additional water vapor the air will hold.

Look up cooling towers.

Answer 3

There is some vapour pressure at room temperature. So vapour is always escaping the surface and if there is a draught to take away the saturated air more will escape to maintain the VP equilibrium.

Answer 4

Its about Partial Pressure of droplets, it is at room temp higher than that of the vapor pressure in the air.

Answer 5

Friction of air on the surface.

Answer 6

i dont know boy!