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What is a Phase Diagram ?

Phase diagrams depict how the primary phases (solid, liquid, gas) of a substance change as the pressure, volume and temperature of the system change.

Phase diagrams are constructed by plotting the following :

1. Temperature as a function of the molar volume (T-

)

2. Pressure as a function of the molar volume (P-

)

3. Pressure as a function of temperature (P-T)

Note : The molar volume,

, is the volume per mole :

The specific volume,

, is the volume per unit mass :

Lesson 2B: page 1 of 12

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Ch2, Lesson B, Page 1 - What is a Phase Diagram ?

A phase diagram shows you the phase in which a material exists at a given state.
This allows you to pack an enormous amount of information about boiling, melting and other phase changes into a nice compact form.
We will only consider phase diagrams for pure substances in this chapter.
With a little practice, I think you will find that phase diagrams will help you gain a much deeper understanding of phases and phase changes.
There are an amazingly large number of different types of phase diagrams. But, we’re going to start out with the big three: T-V, P-V and P-T Diagrams.
It makes sense to start with these three phase diagrams because you are already familiar with the concepts of temperature, pressure and volume.
But what does the wiggly line over the V mean ? Well, the wiggly line means that we are referring to the MOLAR volume.
The molar volume is the volume per mole or just the total volume of the system divided by the number of moles in the system.
The specific volume is represented by a V with a carat, or “hat,” over it. It is just the volume per unit MASS of the system.
So, why bother with molar and specific volume ? Why not just use plain old volume ?
If we used ordinary volume, then we would need one phase diagram for a system that contained 2 kg of water and another for a system that contains 0.2 kg of water.
Why ? Consider a system with a volume of 2 liters that contained 2 kg of water. Nothing remarkable about that since the density of liquid water at atmospheric pressure is about 1 kg/L.
But what if this 2L volume contained just 0.2 kg of water also at atmospheric pressure ? Could all of the water be a liquid ? No, the density is too low. Some of the water would be in a different phase…the gas phase !
So, the phase depends on the SIZE of the system ! This is not very convenient or helpful !
This problem is fixed by using either the molar or specific volume instead of the total volume. This way a single phase diagram can be used regardless of the size of the system.
Cool. So, for example, water has just one P-T Diagram.
Ok, so let’s learn a few terms that will make it easier for us to discuss phase diagrams.

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