What Is the Difference Between Triple Point and Melting Point?

[Paul Snasel]

Hello,

Im new here and I hope someone of you can answer this probably trivial question. I tried to find the answer in many of phyisc/termodynamic texbooks but in vain.

http://imageshack.us/photo/my-images/62/123002h.jpg/
http://imageshack.us/photo/my-images/62/123002h.jpg/

According to the phase diagram 3 phases (fig. II) of matter can coexist only by conditions of triple point, so when:
- temperature = temperature of triple point
- preassure = preassure of triple point at the same time.
Now let's have mixture of gas and vapour, for example mixture of dry air and watter. In literature or in Moillier diagram (fig. I) are determined 4 possible situations:
- the air is not saturated (area A, fig. I),
- the air is saturated (RH = 100%, fig. I),
- the air is over-saturated: watter coexists in phases liquid + gas (area B, fig. I),
- the air is over-saturated: watter coexists in phases solid + gas (area C, fig. I),
- the air is over-saturated: watter coexists in phases solid + liquid + gas (area D, fig. I).
It depends on relation of temperature of mixture to temperature "0°C", which of three possible situations in area of over-saturated air comes. There is alwas mentioned probably rounded value of temperature "0°C" in literature or in Mollier diagram, but without further explanations. So I am asking, is it temperature of triple point T_tp or temperature of melting T₁₂(p)? Or is the problem more complicated? I know, that difference between value of T_tp and T₁₂(p) of watter is negligible, but there are matters where the difference is significant.

Ill appreciate any advice or suggestion. I am sorry for my bad english.

Best regards
Paul

 

[mfb]

The melting point is pressure-dependent. If you want to have all 3 phases at the same time (in equilibrium), you are at the triple point, which is equal to the melting point at the pressure of the triple point. In addition, it is the boiling point at this pressure and the point where sublimation is in equilibrium.

 

[haruspex]

I was taught that NTP is 0C and 760mm Hg, which is not quite the same as the triple point.
Mind you, that was a long time ago.