From: Geoff Ruth (gruth@uclink4.berkeley.edu)
Date: Tue Dec 05 2000 - 11:47:30 PST
Message-Id: <v04011702b652f5e88e14@[192.168.1.101]> Date: Tue, 5 Dec 2000 11:47:30 -0800 From: Geoff Ruth <gruth@uclink4.berkeley.edu> Subject: Re: pinhole sublimation
Aaah, finally an answer that makes conceptual sense to me! According to
your explanation, there should be few nonpolar molecules that can enter
into the liquid phase at room temperature.
For N2 or O2 or He, I'd predict that their triple point on a phase diagram
would be at a lower pressure than for a polar molecules of similar size.
No? Can you explain why all of these 3 do go through a liquid phase rather
than subliming?
-Geoff
>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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