Originally Posted by anachronism
Suddenly this thread got interesting.
Somehow I have gone the entire span of my life from learning that water boils at much lower temps as air pressure decreases (you learn this one in Colorado pretty early, generally the first time you follow one of grandma's recipes to try and make cookies) to now....
Without ever realizing that altitude doesn't have a similar effect on the freezing point.
Can anybody explain to me why?
Originally Posted by oldgoat
The lower the atmospheric pressure the less vapor pressure of the liquid is required for the liquid to boil, which means it can boil at a lower temperature.
That's correct. But we're still left with the question of why this is the case and why the effect of pressure on freezing temperature is so much less. Here's a non-technical explanation (no equations!):
When you boil water, it expands a lot. In expanding against a fixed external pressure, the system does work on the surroundings. If you reduce the external pressure, less of this pressure-volume (p-V) work needs to be done, so the water becomes easier to boil (boils at a lower temperature). The same thing happens when you freeze water -- it also expands, which is why it becomes easier to freeze (freezes at a higher temperature) when the pressure is lower. However, the difference in volume between liquid and gaseous water is huge, so lowering the pressure significantly reduces the p-V work associated with the phase change, and thus has a significant effect on the boiling point. By contrast, the difference in volume between ice and liquid water is far smaller, so the effect of external pressure on freezing point is correspondingly smaller.
If you go to this site and read the section on "effect of pressure," it will tell you more:
[Now, if you want a fundamental explanation for why needing to do less p-V work on the surroundings makes water easier to boil, ultimately it's all about the entropy, since what we're really doing is finding the equilibrium state of water that maximizes the entropy of the universe (Second Law of Thermodynamics). If the the entropy is already maximized in that state, it stays there (unless you do something to it, like heat or cool it). If not, it will spontaenously switch to the state at which the entropy of the universe is maximized, unless the system is constrained in some way (and yes, you need a perturbation, otherwise you can get, for instance, a metastable state like supercooled water). At 1 atm and 99 C, the entropy of the universe is maximized when the water is in a liquid state; at 0.5 atm and 99 C, the entropy of the universe is maximized when the water is in a gaseous state. The p-V work term in the thermodynamic equations is what captures the effect, on the entropy of the universe, of what I described above.]
Hope this helps!
Edited by chemist - 10/31/14 at 6:41pm