Chapter 4: Phases and Phase Equilibria

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[annamarie.3593]

the text states: "increases in the prevailing atmospheric pressure decreases the melting point."

This doesn't seem to make intuitive sense to me and I don't know understand the rationale behind this. Please explain!

[mcat_premed3832]

This is a direct application of Le Chatelier's principle. Liquid water is unusual because when it is frozen, it expands (thus ice floats in liquid water). So if you apply pressure to the solid form, it will preferentially shift to the liquid form to relieve the stress.

In the boiling/melting point video on our site, the example is given of ice skates: they apply pressure to solid ice to create a thin layer of liquid on the solid ice which is therefore slippery and thus the skate can glide. If there was no melting, there would be too much resistance to skate.