Altitude and MP

[justadream]

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I read somewhere in TPR that higher altitude (lower pressure) = HIGHER MP for water?

Why is this?*

*I mean, I can see it from looking at the diagram with solid/liquid/gas but if someone can offer a more conceptual explanation, that would be great!

 

[Czarcasm]

I read somewhere in TPR that higher altitude (lower pressure) = HIGHER MP for water?

Why is this?*

*I mean, I can see it from looking at the diagram with solid/liquid/gas but if someone can offer a more conceptual explanation, that would be great!

Decreasing pressure favors the less dense form. You're essentially applying less force on a given area. Water is very unusual in that the less dense form is actually the solid instead of the liquid. On a molecular level, the reason for this is that hydrogen bonding is maximally arranged in ice, forming a nice crystallized lattice structure. (In doing so however, this increasing the volume as a whole, making it less dense than water). So, by decreasing pressure, more of this organized lattice is forming and this is a very stabilizing feature (maximizes the amount of hydrogen bonding). As a result, more energy (heat) is needed to disrupt this stability to get it to melt. It might help to look again at a phase diagram to help clear this up.

 

[sillyjoe]

Decreasing pressure favors the less dense form. You're essentially applying less force on a given area. Water is very unusual in that the less dense form is actually the solid instead of the liquid. On a molecular level, the reason for this is that hydrogen bonding is maximally arranged in ice, forming a nice crystallized lattice structure. (In doing so however, this increasing the volume as a whole, making it less dense than water). So, by decreasing pressure, more of this organized lattice is forming and this is a very stabilizing feature (maximizes the amount of hydrogen bonding). As a result, more energy (heat) is needed to disrupt this stability to get it to melt. It might help to look again at a phase diagram to help clear this up.

TBR says you should know for the MCAT that water is the densest at 4 degrees centigrade. As @Czarcasm pointed out, water is unusual in that it is densest as a liquid.