Ask A Scientist Chemistry Archive

 
name         Haythm I.
status       educator
 age          20s

Question -   At which temperature the evaporation of water begins?
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There is no specific temperature at which the evaporation of water begins
or stops. The pressure of water vapor in equilibrium with either solid
water (ice) or liquid water increases continuously with increasing
temperature. For example, at -10 C the vapor pressure of water above ice is
1.950 mm Hg. At the melting point, 0 C the vapor pressure if both ice and
liquid water are equal and is 4.579 mm Hg. At 4 C where liquid water has
its maximum density the vapor pressure is 6.101 mm Hg. At 25 C the vapor
pressure is 23.756 mm Hg. And of course at 100 C the vapor pressure is 
760.0 mm Hg
(= 1 atm the "normal" boiling point of water). At 374.1 C the vapor 
pressure of
water is 978.3 mm Hg. At this temperature the density of  liquid water is
equal to the density of water vapor (called the critical point). Above that
temperature and pressure water is a single fluid phase, neither liquid nor
vapor.

Vince Calder
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Haythm,

If you remember the graph depicting Maxwell's distribution of kinetic energy,
you will realize that in this theory there will always be molecules moving at
high enough speeds to escape the intermolecular forces of the liquid phase.
Thus, the answer is that everything, including solids must have a vapor above
it. However, at a certain point, we do have to say that the vapor pressure is
not appreciable and so we arbitrarily draw the line somewhere. So, in answer
to your question, water evaporates at all temperatures (you can even prove
that solid water sublimes by simply putting it in a "no-frost" freezer that
blows air too strongly - the ice will eventually disappear).

Greg (Roberto Gregorius)
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