Phase Diagrams
• Phase diagram: plot of pressure vs. Temperature
summarizing all equilibria between phases.
• Given a temperature and pressure, phase diagrams tell
us which phase will exist.
• Features of a phase diagram:
– Triple point: temperature and pressure at which all three phases are in
equilibrium.
– Vapor-pressure curve: generally as pressure increases, temperature increases.
– Critical point: critical temperature and pressure for the gas.
– Melting point curve: as pressure increases, the solid phase is favored if the solid is
more dense than the liquid.
– Normal melting point: melting point at 1 atm.
video
Phase Diagrams
• Any temperature and pressure combination not on a
curve represents a single phase.
Phase Diagrams
The Phase Diagrams of H2O and CO2
• Water:
–
–
–
–
–
The melting point curve slopes to the left because ice is less dense than water.
Triple point occurs at 0.0098C and 4.58 mmHg.
Normal melting (freezing) point is 0C.
Normal boiling point is 100C.
Critical point is 374C and 218 atm.
• Carbon Dioxide:
– Triple point occurs at -56.4C and 5.11 atm.
– Normal sublimation point is -78.5C. (At 1 atm CO2 sublimes it does not melt.)
– Critical point occurs at 31.1C and 73 atm.
Phase Diagrams
The Phase Diagrams of H2O and CO2
• Critical Point: indicates the critical
temperature and critical pressure
• Critical Temperature: temperature above
which the substance cannot exist in the liquid
state
• Critical Pressure: lowest pressure at which the
substance can exist as a liquid at the critical
temperature
Triple Point of
Water
Substance
T (K)
P (kPa*)
Acetylene
192.4
120
Ammonia
195.40
6.076
Argon
83.81
68.9
Butane[5]
134.6
7 × 10−4
Carbon (graphite)
4765
10132
Carbon dioxide
216.55
517
Carbon monoxide
68.10
15.37
Chloroform[6]
175.43
0.870
Deuterium
18.63
17.1
Ethane
89.89
8 × 10−4
Ethanol[7]
150
4.3 × 10−7
Ethylene
104.0
0.12
Formic acid[8]
281.40
2.2
• At the critical point C, the properties of the
gas and liquid phases approach one another,
resulting in only one phase: a homogeneous
supercritical fluid
It can diffuse through solids like a
gas, and dissolve materials like a
liquid
Supercritical CO2 Used to Decaffeinate Coffee
is any substance at a
temperature and pressure
above its critical point
Copyright 1999, PRENTICE
HALL
Chapter 11
8
Applications
•
•
•
•
Decaffeinating coffee
Solvents for dry-cleaning
Refrigeration
Biodiesel production
• Ice melts at a higher temperature with
decreasing pressure
Application
• Freeze drying food: freeze food and then
lower pressure so ice sublimes