Phase Diagrams | OAT
Phase Diagrams
• In the last section, we looked at
phase changes under constant
pressure
• But when pressure is varying, we
look at phase diagrams
•
Wherever there’s a line between
two phases, those two phases are
in equilibrium
Phase Change Equilibrium
At the interface between the solid and liquid
phases, the two are in equilibrium.
Phase Diagram for H2O
•
•
A substance’s phase diagrams
tells us its phase at various
temperatures and pressures
o i.e. at high temperatures
and low pressures, H2O
will be a gas
The phase diagram also helps us
pinpoint phase changes
Sublimation of H2O
At a low pressure, H2O can transition straight
from a solid to a gas during a process called
sublimation. The reverse process is called
deposition.
•
•
The point where all 3 boundaries
come together is called the triple
point
o At the triple point, all 3
phases are in equilibrium
Also, notice that the boundary
between the liquid and gaseous
phases ends early
o The end of that line is
called the critical point
and it marks the critical
pressure and critical
temperature
The Critical Point
The critical point marks the critical pressure and
the critical temperature (arrows).
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© 2017 J Co Review, Inc., Accessed by Guest on 06-18-2017
Phase Diagrams | OAT
•
Beyond the critical pressure and
temperature, the gaseous and
liquid phases become one phase,
known as a supercritical fluid,
which has both gaseous and
liquid-like properties
Phase Diagrams for other Substances
• Solid H2O is unique because
increasing its pressure at
constant temperature will cause
it to become a liquid:
Increasing the Pressure of Solid CO2
When the pressure of solid CO2 is increased, it
serves to re-enforce its compact, solid structure.
•
So then why does ice melt when
we compress it?
o Because water’s liquid
phase is denser than its
solid phase!
Increasing the Pressure of Ice
Increasing the pressure of ice will cause it to
melt to liquid water.
•
•
This is how ice skating works
o The skate’s blade allows
a skater to exert the full
force of his/her weight
over a small area of ice,
thereby melting it
o The skater then slides
across the liquid water
For most other chemicals,
increasing the pressure of the
substance in its solid phase will
only re-enforce its solidity
o i.e. CO2
Liquid and Solid Phases of Water
H2O is denser as a liquid than it is as a solid.
When ice is compacted, it therefore reverts to its
denser phase- liquid water.
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© 2017 J Co Review, Inc., Accessed by Guest on 06-18-2017