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Critical points are defined (by wikipedia) as the "end-point of the phase equilibrium curve".

I would say this diagram therefore has three critical points. But this is unnatural. I doubt its true.

Other curves are different from the ones with critical points because "they can run to infinity". But I think thats ridiculous because its physically impossible.

I don't know if I'm interpreting PDs correctly but I think that what defines that you are on a phase in these diagrams is that if you travel along a horizontal line you meet with a phase equilibrium curve.

If you don't then you were either on a single phase the whole time or on a sort of "critical phase".

I don't like it though. For example if you travel along a horizontal line that wont intersect the L-V curve, how would you know when the LV critical phase is present? Regions are not clearly defined here, except for monoclinic.

Whats wrong with my ideas?

How many critical points does this diagram really have and why?


  • $\begingroup$ reg your example , i think the problem is a practical one, creating a large P and/or T is not easy and might be useless. So the curves shown usually have the portion thats useful for the study. 3 is the right amount of critical points in given curve. Also this becomes very obvious ,say in a binary PD . $\endgroup$
    – Gowtham
    Mar 11, 2015 at 18:10

1 Answer 1


This phase diagram has one critical point - it is where the line separating the liquid and gas comes to an end - it is now one fluid. So, you can go from liquid to gas (or vice versa) by swinging T,P around the critical point without undergoing a first order phase transition.

The points with arrows pointing to them are triple points, which are also special points, but different from the critical point. Movement away from a triple point means that one (or more) of the phases have to undergo a first order phase transition to the stable phase(s).

Thermodynamically, these are very different things.

  • $\begingroup$ Hey, but why are the other curves cut off too? Is it not supposed to be this way? $\endgroup$
    – DLV
    Mar 11, 2015 at 19:32
  • 2
    $\begingroup$ Sloppy drawing of the figure. They extend to infinity and beyond. Probably should have been drawn to the surrounding box, or have arrows on them. Often the critical point is a filled circle big enough that it is obvious that the liquid/gas line is really supposed to end there. $\endgroup$
    – Jon Custer
    Mar 11, 2015 at 19:48

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