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Q & A: boiling versus evaporation

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Most recent answer: 02/03/2017
Q:
normally the boiling point of water is 100 degree celsiusbut the water in water bodies likes lakes,riversand seas the water evoperates lower than boiling point (40 to 50 degree celsius.why?what is the reason?
- R.SUPRIYA (age 16)
MADANAPALLI,AP,INDIA
A:

The ordinary boiling point of water is the temperature at which a gas of pure water vapor at atmospheric pressure is just as stable as liquid water. As many molecules will flow from the liquid to the vapor as flow back. At just slightly higher temperature the net flow is from liquid to vapor. So bubbles of vapor that form in the liquid grow. That's boiling.

At lower temperature, however, the liquid can still be in equilibrium with some vapor, just at lower vapor pressure. Molecules still shake loose fro the liquid and join the gas, just at lower rate. So that's why in equilibrium, where the liquid-->gas rate equals the gas-->liquid rate, the vapor pressure must be less than atmospheric pressure. 

Typically, the atmosphere has less water vapor in it than it would have in equilibrium, so the net flow of molecules at the surface goes liquid-->gas. That's evaporation. It doesn't occur at bubbles inside the liquid because there the water molecules make up ~100% of the gas, not just a small fraction.

Why doesn't the atmosphere get enough water vapor in it to reach equilibrium with liquid water, so that net evaporation would cease? The Earth is mainly heated by sunlight at the surface and cooled by radiating infrared to space from the upper atmosphere. So heat is flowing from the surface on out, generally making the surface hotter than the higher atmosphere. So water tends to condense in the upper atmosphere, from which it rains down as liquid. Then the liquid is reheated by sunlight and evaporates again. The whole thing stays out of equilibrium because of the flow of energy in via sunlight.

Mike W.


(published on 02/03/2017)

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