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Name: Haythm I.
Status: educator
Age: 20s
Location: N/A
Country: N/A
Date: 11/26/2004


Question:
At which temperature the evaporation of water begins?


Replies:
Dear Haythm,

Evaporation is a surface effect. The temperature of a body of water represents an AVERAGE value of the kinetic energy of the water molecules in the water. Some of those molecules have a "higher than average" kinetic energy and break free of the surface to become water vapor. As this happens over time, the remaining body of water becomes slightly cooler (as water molecules with higher kinetic energy have "left"). That is why sweat evaporating from your body provides a mechanism to cool you off.

This occurs at any temperature of water between the melting point and the boiling point -- with more evaporation occurring with a body of water that has a higher average temperature (that is, closer to the boiling point). It can even happen below the freezing point (via sublimation -- when water goes directly from a solid (ice) to water vapor) and this explains why the ice cubes in your freezer sometimes seem to "evaporate".

All of this is assuming that you are at standard pressure of 1 atmosphere. As the atmospheric pressure changes, water will boil and freeze at different temperatures.

Here is another answer in the Newton Archives that also provides some useful information:

http://www.newton.dep.anl.gov/askasci/phy00/phy00130.htm

Todd Clark, Office of Science
U.S. Department of Energy


Haythm -

Evaporation does not have any sharp beginning point. It just gets slower and slower as the temperature goes down. Even ice evaporates. For each 10 degrees C colder, it gets a factor of two or so slower. Actually, the colder it gets, the more it slows down for each 10 degrees C.

For slow evaporation which does not make the water much cooler than the surrounding environment, the evaporation rate is proportional to the vapor pressure of water at that temperature.

It is also proportional to the exchange rate of the air immediately over the water. The water vapor must escape to elsewhere by either air movement or diffusion, or else it will build up in concentration until it is in equilibrium with the liquid or solid, and then evaporation has been stopped. Evaporation rate is very situation-dependent in that sense. So we are talking about approximations when we compare rates at different temperatures.

So find a table of the vapor pressure of water from 0 degrees C down to perhaps -100 C. Those numbers will be proportional to the evaporation rate of ice at those temperatures.

From my CRC reference book:
Temperature  Pressure
(degrees C)   (mmHg)
   -90 C     0.00007 mmHg
   -80 C     0.0004  (factor 5 per 10C)
   -70 C     0.0019
   -60 C     0.008   (factor 4 per 10C)
   -50 C     0.030
   -40 C     0.097   (factor of 3  per 10C)
   -30 C     0.29
   -20 C     0.78
   -10 C     1.9
     0 C     4.6  (freezing)
    10 C     9.2     (factor of 2  per 10C)
    20 C    17.5
    30 C    32       (factor less than 2 per 10C)
    ...
   100 C   760 mmHg (boiling)

Seeing the 4's in the table above, I can say that: At -80 C ice evaporates about 10,000 times slower than it does at 0 C.

And in a freezer at -10C, the ice should evaporate about 10 times slower than water at room temperature.

Jim Swenson



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