Triple Point

[rskhan29]

I'm confused over when a substance can be in equillibrium with all three "common" phases (liquid/gas/solid). Most books say that this occurs at the triple point, but can't it occur at any point? For example, the melting point is when the vapour pressure of the liquid phase and pure solid are equal, suggesting that all three phases are in equillibrium, albeit at an equillibrium where the gas phase is very little. Any help?

I think whats messing me up is the idea of melting point. Its said that as we approach melting point, the liquid vapour pressure rises to match and then exceed the solid vapour pressure. Can anyone help me interpret this?

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[engineeredout]

I'm confused over when a substance can be in equillibrium with all three "common" phases (liquid/gas/solid). Most books say that this occurs at the triple point, but can't it occur at any point? For example, the melting point is when the vapour pressure of the liquid phase and pure solid are equal, suggesting that all three phases are in equillibrium, albeit at an equillibrium where the gas phase is very little. Any help?

Uh what? Vapor pressure of solid? When the vapor pressure of the liquid phase equals the vapor pressure of atmospheric, the substance vaporizes. Maybe you're thinking of sublimation when the solid turns directly into a vapor. There is no liquid phase within that. If you look at the phase diagram below, you can see that. I mean yeah you're right when you use that definition of melting point, but why is it that you think there is a gas phase in there?


There is only one triple point between the three standard phases (SLG) of a substance. It is at this point that the pressure and temperature are exactly right.

Notice how if you start screwing with pressure and/or temperature and deviate from the triple point, you won't be able to get back to it at any other point.




Oh by the way, if you're considering a substance that has multiple phases, the number of triple points is pC3 where p is the number of phases. 3C3 = 1.

 

[rskhan29]

Hey, thanks for the reply.
I'm using the definition for melting point given by Examkrackers. According to EK, a solid melts when the liquid vapour pressure equals the solid vapour pressure. Above the melting point, liquid vapour pressure exceeds the solid vapour pressure. Below the melting point, the liquid vapour pressure is less than the solids vapour pressure.

I find it to be a bit confusing, and I can't find a similar defintion in any of my gen chem books.
What got me to ask this question was an EK practice question that asked:
Can ice, liquid h2o, and h2o vapour exist in equillibrium at 0 celsius and 1atm?
Answer: Yes, because the ice and water can have an equillibrium with the vapour phase regardless of the pressure or the temperature.