[Changster101]

Hi,
I'm confused on what it means when the vapor pressure of solid is equal to the vapor pressure of liquid. In my Barron's SAT Chemistry book, they state, "when the VP of a solid is equal to the VP of a liquid, melting and freezing take place in equilibrium with each other." What I'm confused about is the fact that vapor pressure of a solid has to do with sublimation, when VP = Atmospheric Pressure. What does the vapor pressure of a solid (sublimation), have to do with the melting point and freezing point?

[Borek]

vapor pressure of a solid has to do with sublimation, when VP = Atmospheric Pressure

No. Sublimation doesn't require VP to equal atmospheric pressure. It requires reasonably high VP to make the process fast enough, but doesn't call for any particular VP (as BP does). BP is a function of external (atmospheric pressure) but you don't need BP to understand what they are talking about.

Imagine having both solid and liquid in vacuum. If the VP over one of the phases is higher than VP over the other phase, what will happen? Think in terms of evaporation and condensation. Will there be mass transfer between phases? Can you think of a condition that would make both phases coexist?