From: Marc Afifi (marc_afifi@yahoo.com)
Date: Tue Dec 05 2000 - 11:36:10 PST
Message-ID: <20001205193610.26950.qmail@web209.mail.yahoo.com> Date: Tue, 5 Dec 2000 11:36:10 -0800 (PST) From: Marc Afifi <marc_afifi@yahoo.com> Subject: Re: pinhole sublimation
I'll try this one.
In fact you can have liquid CO2 and liquid I2 at room
temperature, but not for long since they will both
boil away due to their extremely high vapor pressures.
Both I2 and CO2 are nonpolar molecules and thus their
molecules are attracted to each other by London Forces
(weak). These forces are not very effective for
molecules moving at high speeds. Therefore the liquid
phase does not readily form unless the pressure is
high enough to squeeze the molecules closer together
so that the weak attractive forces will be effective.
In the solid phase the average velocity of the
molecules is slow enough that these attractive forces
can in fact cause the molecules to condense to the
solid phase. They don't condense to the liquid phase
because they are moving too slowly to be a liquid.
Some of the molecules, however, are still moving at
very high speeds (remember, temperature is the average
kinetic energy), high enough to escape the weak
attractive forces. This is why the solid will
eventually sublimate away -- the fast moving molecules
escape leaving the slower ones behind.
Water evaporates for much the same reason except that
the water molecule is so polar that there are stong
forces of attraction that are effective even at higher
temperatures. Boiling is a different story. Boiling
occurs when the vapor pressure of the liquid equals
the atmospheric pressure. Water can be made to boil at
room temperature by lowering the atmospheric pressure.
In fact, liquid water in outer space will first boil
away until all that's left is ice which will then
sublimate away.
Hope this helps.
-Marc
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