# How many critical points? Why?

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?

Thanks.

• 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 . Commented Mar 11, 2015 at 18:10