MadSci Network: Physics
Query:

Re: Why does melting point decrease in a vacuum?

Date: Sat Jan 18 15:06:27 2003
Posted By: Allan Harvey, Chemical Engineer
Area of science: Physics
ID: 1042635471.Ph
Message:

Where did you get the idea that melting point decreases in a vacuum?  
Boiling point certainly does, but melting point is something different.  
Melting happens when the temperature gets high enough that the motion of 
the molecules overcomes packing into a solid structure, so the liquid 
becomes more stable.  The pressure has only a small effect on that.

I can think of 4 ways a vacuum *might* seem to affect a melting point:

1) The pressure does have a *very small* effect on the solid/liquid 
equilibrium.  For water, going from atmospheric pressure to near a vacuum 
*raises* the melting temperature by about 0.01 K.  For most other 
substances (where the solid is more dense than the liquid) the effect is 
in the reverse direction (so that vacuum will lower the melting point), 
but the effect is still small, at least for the small pressure difference 
between atmospheric and vacuum.  For the explanation of this effect, look 
in a physical chemistry textbook under "Clapeyron equation."

2) For some substances (especially metals), air can react with the solid 
and form a compound that will have a different melting point than the pure 
solid.  This could affect the melting point in either direction, and doing 
it in a vacuum would get rid of this effect.  Some high-temperature work 
with metals is done in a vacuum for this reason.

3) Solids can also "boil" since they have a vapor pressure (this is called 
sublimation).  So, for example, if you make vacuum above some ice, it will 
eventually sublime and go away (like snow can evaporate on a dry sunny day 
even if the temperature is below freezing).  But this is not melting.

4) For some compounds, if you heat them they will decompose before they 
melt.  If you are under vacuum, the products of the decomposition will get 
carried away faster, so the substance will disappear faster.  But this is 
not true melting either.

About your other question, the prediction of melting points is in general 
extremely difficult, even more so than prediction of other properties.  
This is because it depends strongly on how the molecule can pack into a 
regular lattice arrangement, and that is very difficult to predict (except 
for simple molecules like argon).  People have put in a lot of research to 
come up with schemes to predict the melting point based on molecular 
structure, but frankly none of these is very reliable (though it can work 
for a series of very similar compounds -- you can predict long alkanes 
based on shorter alkanes for example).  If people have an important 
material they need to know the melting point of, measuring it 
experimentally is still the way to go.  Measuring phase diagrams is still 
an important (and underappreciated) part of science.


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