# Boundary between supercritical fluid and gaseous phase

As far as I understand, going around the critical point, it is possible to transform a liquid into a gas without seeing a phase transition.

Does this also mean that there is no clear separation between a gaseous phase and a supercritical fluid ? This is more or less what can be understood for the dotted line between the two that usually appears in a phase diagram such as the following:

So my question is: Is there a standard physical measure for deciding whether a given fluid is in gaseous phase or in supercritical state? Alternatively, is the boundary between the two clear-cut or somewhat fuzzy?

• "Supercritical" fluid is just a name given to the liquid/gas in that regime and has no underlying meaning, and the edged lines are just some guides to the eye. I do not know if there are standardized criteria for using the word. But keep in mind that any criteria is like deciding what wavelengths of light to call "green". It could be useful to agree, but there is nothing physical about it. – BebopButUnsteady Aug 23 '13 at 20:00

By the definition of supercritical fluid, the state are clearly defined if the critical point is known. As far as the measured temperature $T$ and pressure $P$ that is higher than the critical point, you know it is supercritical fluid. There are no clear separation between gaseous phase nor liquid phase, in the sense that by changing $T$ and $P$ only by a little bit, it will go into another "phase".

In comparison, existence of phase transition can be determined locally without any reference points. If the system state is extremely close to the transition line, the measurable physical quantity will change abruptly with any small change in the control parameters in the direction of crossing the phase transition line.

• Thanks for your answer, I will accept that. @BebopButUnsteady comment was also quite useful. I have made some more web research to understand a bit more about this. I will post an answer to sum up my findings. Please correct me if I am wrong. – minar Aug 27 '13 at 19:13

After some more reading, I think I better understand the issue. I summarize my findings here, please correct me if something is wrong.

In the subcritical zone, there is a phase transition between liquid and vapor when the temperature and pressure lie on the phase boundary. The phase transition is characterized by the fact that liquid and vapor exist simultaneously in equilibrium.

The key experiment to show the critical point is to keep the liquid/vapor in equilibrium, while slowly moving to the right along the phase boundary (simultaneously increasing the temperature and pressure). At some point (the critical point), the two phases become identical and the resulting fluid is called supercritical.

Since we were moving along the phase boundary, the supercritical fluid appears when both the temperature and pressure become supercritical, which justify the standard definition of a supercritical fluid. The following 3D diagram from http://en.wikipedia.org/wiki/Phase_diagram is quite useful for visualizing the critical point.

However, the transition from liquid/vapor to supercritical is only clear when following the phase boundary. If one goes around the supercritical point, nothing visible appears and one goes smoothly from liquid to vapor. As a consequence, there is no clear physical delimitation of the supercritical zone, the definition is just conventional.

Moreover, if one were to seek a delimitation somewhere, the Windon line between liquid-like and gaz-like behavior (see http://www.nature.com/nphys/journal/v6/n7/full/nphys1683.html) would be a better candidate.

• You seems understand it better now :) – unsym Aug 28 '13 at 3:57