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Q & A: what happens at a triple point?

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Most recent answer: 10/02/2013
Q:
At the triple point of water (or any other substance), what determines the specific state of matter that each individual atom or molecule is in?
- Daniel Hoss (age 17)
Normal, IL USA
A:

That's a great question.. The general rule is that at fixed temperature and pressure the material will end up in a form which gives it  the lowest free energy, "G". At the triple point the liquid, solid, and gas have the same G per molecule. So that makes the question a little tricky.

Even at an ordinary two-phase transition point  it's sort of an accident of the prior history how much thermal energy (or more precisely, enthalpy) happens to be in the material, and that determines the fraction of the molecules in each of the two phases. Given that historically determined fraction, there may be some slight external effect that fixes what part is where. For example, for water on earth gravity makes the solid part end up on top because it's slightly less dense. All other things being equal, however, the overall free energy can be lowered slightly by having the entire material be all one phase or all the other. That's because there's some extra free energy at the surface, the same thing that causes surface tension.So if somehow the material all gets to be one phase or the other, it will stay that way so long as it's at the phase transition point.

During the period where more than one phase is present, individual molecules can leave the liquid and go to the solid and vice-versa. The equilibrium between them is only maintained as a sort of statistical average of those individual molecules' fluctuations.

The triple point is more complicated, since even if you've specified, for example, that the enthalpy is just the value that the liquid would have, you could get the same value by converting some to solid (lower enthalpy) and some to gas (higher enthalpy). However, we still have the general rule that it costs some G to make any surface between two different phases. So in that case the material should tend to form just two phases for any particular value of the enthalpy, to minimize that surface G. Then if somehow it gets to a one-phase state, it should lock in and stay that way.

 

Mike W.


(published on 10/02/2013)

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