Pressure and temperature phase diagrams

[laser1]

Homework Statement

NA

Relevant Equations

NA

"Critical pressure: minimum P needed to liquefy at the critical T." is the definition in my notes. Why is the word "minimum" used? As any higher P at the critical T would make the phase a supercritical fluid.

 

[Lnewqban]

A fluid in supercritical conditions that is above the critical pressure and temperature can’t be liquefied.

Copied from
https://en.m.wikipedia.org/wiki/Critical_point_(thermodynamics)#Overview

“Above the critical point there exists a state of matter that is continuously connected with (can be transformed without phase transition into) both the liquid and the gaseous state. It is called supercritical fluid. The common textbook knowledge that all distinction between liquid and vapor disappears beyond the critical point has been challenged by Fisher and Widom, who identified a p–T line that separates states with different asymptotic statistical properties (Fisher–Widom line).”

 

[laser1]

A fluid in supercritical conditions that is above the critical pressure and temperature can’t be liquefied.

yeah that's what I'm saying, hence, is the word "minimum" in the definition redundant?

 

[jbriggs444]

View attachment 353410

If you look at the diagram you posted and concentrate on the area to the left of $T_\text{cr}$ you can see the [compressible] liquid phase exists right next to $T_\text{cr}$ through a range of pressures.

The minimum pressure at which the liquid phase is seen just to the left of $T_\text{cr}$ is the critical pressure.

I do not think that we need to quibble about what happens exactly at $T_\text{cr}$ or try to distinguish between a compressible liquid and a supercritical fluid. Just follow the line of critical temperature down until you are no longer adjacent to a region in the liquid state.

 

[laser1]

If you look at the diagram you posted and concentrate on the area to the left of $T_\text{cr}$ you can see the [compressible] liquid phase exists right next to $T_\text{cr}$ through a range of pressures.

The minimum pressure at which the liquid phase is seen just to the left of $T_\text{cr}$ is the critical pressure.

I do not think that we need to quibble about what happens exactly at $T_\text{cr}$ or try to distinguish between a compressible liquid and a supercritical fluid. Just follow the line of critical temperature down until you are no longer adjacent to a region in the liquid state.

I get what you are saying. But - above the Tc in the diagram, it is either a compressible fluid or supercritical fluid, right? So how can you liquefy these as they are not gases.

 

[jbriggs444]

I get what you are saying. But - above the Tc in the diagram, it is either a compressible fluid or supercritical fluid, right? So how can you liquefy these as they are not gases.

Which is why I suggested you concentrate your attention just to the left of the critical line.

 

[laser1]

Which is why I suggested you concentrate your attention just to the left of the critical line.

Can you liquefy a gas at any other point on the critical temp line?