What is a distinct phase change?

In summary: 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
mech-eng
828
13
"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.
 
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  • #2
Distinct phases means that there is something that is discontinuous, such as density.
 
  • #3
DrClaude said:
Distinct phases means that there is something that is discontinuous, such as density.

Now can you explain how density can be discontinous?

Thank you.
 
  • #4
mech-eng said:
Now can you explain how density can be discontinous?
Think liquid and gas.
 
  • #5
DrClaude said:
Think liquid and gas.

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

Thank you.
 
  • #6
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
DrClaude said:
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.
 
  • #8
mech-eng said:
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.
 
  • #10
hilbert2 said:
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.
 
  • #11
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.
 

1. What is a distinct phase change?

A distinct phase change refers to a physical change in matter where it transitions from one state to another, such as from a solid to a liquid or from a liquid to a gas. This change occurs due to a change in temperature or pressure, and is accompanied by a change in the physical properties of the substance.

2. What are the different types of phase changes?

There are six main types of phase changes: melting, freezing, evaporation, condensation, sublimation, and deposition. Melting and freezing involve a change between solid and liquid states, while evaporation and condensation involve a change between liquid and gas states. Sublimation and deposition involve a change between solid and gas states.

3. How is energy involved in a phase change?

During a phase change, energy is either absorbed or released. When a substance changes from a solid to a liquid or from a liquid to a gas, energy is absorbed in the form of heat. This is known as an endothermic process. On the other hand, when a substance changes from a gas to a liquid or from a liquid to a solid, energy is released in the form of heat. This is known as an exothermic process.

4. What factors can affect a phase change?

The main factor that affects a phase change is temperature. As temperature increases, substances tend to change from solid to liquid to gas. Pressure can also affect phase changes, especially for substances that have a low boiling point. Additionally, the composition and purity of a substance can influence its phase change behavior.

5. Can a substance undergo multiple phase changes?

Yes, a substance can undergo multiple phase changes, especially if the temperature or pressure is continuously changing. For example, water can start as a solid (ice), melt into a liquid, and then evaporate into a gas if the temperature continues to increase. Similarly, it can also start as a gas (water vapor), condense into a liquid, and then freeze into a solid if the temperature decreases.

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