# 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 :
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### Ch 2, 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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