# What is a distinct phase change?

1. Apr 24, 2017

### 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.

2. Apr 24, 2017

### Staff: Mentor

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

3. Apr 24, 2017

### mech-eng

Now can you explain how density can be discontinous?

Thank you.

4. Apr 24, 2017

### Staff: Mentor

Think liquid and gas.

5. Apr 24, 2017

### mech-eng

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

Thank you.

6. Apr 24, 2017

### Staff: Mentor

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.

7. Apr 26, 2017

### mech-eng

How could this situation be represented graphically? Is the discontinuity for properties exactly the same for what we learnt 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.

8. Apr 26, 2017

### hilbert2

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.

9. Apr 27, 2017

### Staff: Mentor

10. Apr 28, 2017

### mech-eng

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.

11. Apr 28, 2017

### 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.