What is a distinct phase change?

[mech-eng]

"At pressures above the critical pressure, there is not a distinct phase change
process. Instead, the specific volume of the substance continually increases, and at all times there is only one phase present."

What does "distinct phase change" mean here? First to understand this I should correctly define the phase

1. Phase is a state of substances ( I am not sure they should be pure or not) having two indipendent property.
2. Every phase of pure substance have different indipendent property for that substance.

Thank you.

 

[DrClaude]

Distinct phases means that there is something that is discontinuous, such as density.

 

[mech-eng]

Distinct phases means that there is something that is discontinuous, such as density.

Now can you explain how density can be discontinous?

Thank you.

 

[DrClaude]

Now can you explain how density can be discontinous?

Think liquid and gas.

 

[mech-eng]

Think liquid and gas.

I cannot get the relationship being liquid or gas and discontinuity of density.

Thank you.

 

[DrClaude]

In a first-order phase transition the substance undergoing the transition has some characteristic property than changes discontinuously. In the case of a liquid becoming a gas, the density is different for the liquid phase and the gas phase. Therefore, this is a distinct phase change: as the phase transformation is happening, there are clearly two distinct phases present, that you can characterize by their densities.

 

[mech-eng]

Distinct phases means that there is something that is discontinuous, such as density.

How could this situation be represented graphically? Is the discontinuity for properties exactly the same for what we learned in discontinuity of functions ie broken graphs? I don't remember whether or not I came up with such graphs in thermodynamics textbooks.

Thank you.

 

[hilbert2]

How could this situation be represented graphically? Is the discontinuity for properties exactly the same for what we learned in discontinuity of functions ie broken graphs? I don't remember whether or not I came up with such graphs in thermodynamics textbooks.

Thank you.

Just plot molar volume as a function of temperature, keeping pressure constant, and you'll get a discontinuous graph at least if the system contains only one pure chemical substance and the conditions are below the critical point.

When approaching the critical point, the discontinuity becomes smaller, and close enough to criticality you can't tell the difference between the liquid and gaseous phases anymore.

 

[DrClaude]

There is a nice picture in the German version of Wikipedia:
https://de.wikipedia.org/wiki/Phasenübergang#Klassifikation_nach_Ehrenfest
The first row is no phase transition, the second row first-order transitions, and the third row second-order phase transitions.

 

[mech-eng]

Just plot molar volume as a function of temperature, keeping pressure constant, and you'll get a discontinuous graph at least if the system contains only one pure chemical substance and the conditions are below the critical point.

When approaching the critical point, the discontinuity becomes smaller, and close enough to criticality you can't tell the difference between the liquid and gaseous phases anymore.

1. Why they should be molar volume instead of specific volumes?

2. I do not know how I can plot molar volumes as a function of temperature, keeping pressure constant. Because I do not have ready molar volume values.

Thank you.

 

[hilbert2]

It can just as well be the specific volume. That kind of plots usually have to be made from exp. data, even though more advanced equations of state such as Redlich-Kwong can model phase transitions to some extent.