1. Jespersen 6th Ed Sec 12.10 Dr. C. Yau Fall 2014
Phase Diagrams
Jespersen 6th Ed
Sec 12.10
Dr. C. Yau
Fall 2014

2. Phase Diagrams
Phase diagrams show the graphical relationships between P and T of a substance that determine its physical state.
Typically, it is a graph with T on the x-axis and P on the y-axis. P T

3. Phase Diagrams Show Transitions of Phases
vapor
liquid
solid

4. Only solid phase exists.
Phase Diagram of Water (Fig p.556)
Only solid phase exists.
Only liquid phase exists.
Along BC line, solid & liquid coexist in equilibrium.
Along BD line, liquid & gas coexist in equilibrium.
Along AB line, solid & gas coexist in equilibrium.

5. What happens at 1 atm as T increases?
Phase Diagram of Water (Fig p.556)
Know how to tell what the normal bp and mp are.
Know the location of...
the Critical Point
The Tc and Pc
the Triple Point.
What happens at 1 atm as T increases?
What happens at 2.15 torr as T increases?
Can water sublime at 1 atm?

6. Phase Diagram of CO2 (Fig. 12.31 p.557)
Critical T of CO2 = 31.1oC
Critical P of CO2 = 72.8 atm
Beyond this point, liquid and gas can no longer be distinguished.
In general a gas can be liquefied by increasing the pressure. Pc Tc
However, above Tc, a gas becomes a “supercritical fluid” which is neither a gas nor liquid.
Supercritical fluid form of CO2 is used to extract caffeine out of coffee. It is removed by warming to room temp.

7. Note the slant of the solid/liquid line.
Phase Diagram of CO2
Where is CO2 in this diagram under normal conditions?
Can we condense CO2 into liquid by cooling it down?
Where is dry ice in this diagram?
Can we melt dry ice?
Note the slant of the solid/liquid line.
When P is increased what happens to the mp?

8. This indicates that the solid is harder to melt at higher P.
Fig p.557
This is consistent with our concept that solids are more densely packed than liquids.
There is less crowding as a solid, so it resists melting into a liquid.
If the solid/liquid line has a positive slope (slants to the right), mp increases with increase in P.
This indicates that the solid is harder to melt at higher P.
This means the solid phase is more dense than the liquid phase.

9. Phase Diagram of Water (Fig. 12.29, p.556)
This is consistent of what we know about water.
Ice is less dense than water. That is why it floats...and why you should not freeze water in a sealed glass container.
Note the slant of the solid/liquid line.
Mp decreases with increase of P.
Solid phase is less dense than liquid phase.

10. Structure of Ice: Why ice is less dense than water.
Ice structure has holes making it less dense than liquid water.
Such a structure gives a maximum amount of H-bonding.

11. Comparison of Water & Ice
at the Molecular Level
Water Ice
This is why ice floats on water.