University College - Chemistry Chem 112A Lecture Notes - Lecture 37: Phase Boundary, Phase Diagram, Boiling Point
13 Dec 2016
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20 April 2016
Lecture 37: Phase Diagrams
Equation of the Day: F = C + 2 - P
I. Phase Diagrams
A. Water Phase Diagram
1. Imagine this is being constructed starting with ice in a cylinder with a movable piston with
the piston resting on the ice and slowly being heated
a. Lines are where phases are in equilibrium and points are as well
B. Phase Diagrams
1. Each phase boundary line represents pressure/temperature values at which 2 phases are in
equilibrium
2. The liquid/vapor line allows us to determine the vapor pressure at any temperature or the
boiling point at any external pressure
3. From the solid/vapor phase boundary, we can determine the vapor pressure of the solid at any
temperature, or the sublimation point at any external pressure
4. From the liquid/solid phase boundary, we can determine the melting or freezing point as a
function of external pressure
a. For water this has a backward slant, so at higher external pressure, liquid water is more
stable than the solid
b. However, this doesn’t ever reach the y-axis because there are polymorphs of ice that are
denser than water (there several ice polymorphs actually, so more lines on the water
phase diagram)
• Ie- Ice 1 then after 2000 atm, Ice 3 is denser than water
5. The Triple Point
a. The point where all 3 phases exist in equilibrium
b. For water, the triple point exists at P = 0.0060 atm (4.6 torr) and T = 0.0098°C
c. The liquid phase cannot exist at pressures below the triple point pressure
6. The Critical Point
a. We can’t distinguish the liquid form the (super dense) vapor here, and we call the
substance a “supercritical fluid”
b. The critical point for water exists at T = 374 °C and P = 218 atm
c. The liquid phase cannot exist at temperatures above the critical point temperature
7. Phase rule
a. In the middle of boundary lines, there are 2 degrees of freedom (both T and P can vary)
b. On a phase boundary line, there is 1 degree of freedom (given either pressure or
temperature, you can determine the other)
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