Chem 2 - Phase Transitions I

1. Phase Transitions
Pt. 1
By Dr. Shawn P. Shields
This work is licensed by Shawn P. Shields-Maxwell, Ph. D., under a Creative Commons Attribution-NonCommercial-
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2. Recall: Our Central Question
The physical state of matter depends on
the balance between the kinetic energies
of particles
and the strength of their interparticle
attractions (intermolecular forces).
Why is a particular substance a gas, liquid,
or solid (at a given temperature)?

3. Phases of Matter and IM Forces
Substances that are
Gases at room temperature (RT) have
relatively low intermolecular attractions
Liquids at RT have intermediate strengths
of intermolecular attractions
Solids at RT have the strongest
intermolecular attractions

4. Phase Transitions are Physical Changes
Think about an ice cube at a constant
1 atm pressure and 5 C…
Hydrogen
bonds

5. Phase Transitions are Physical Changes
Below 0C (at low enough
temperature), the kinetic
energy of the water
molecules is not high
enough to disrupt the
hydrogen bonds.

6. Phase Transitions are Physical Changes
Increase the temperature to
25C (const P)…
Heat flows into the ice cube
and the kinetic energy of the
water molecules increases.
The ice cube melts.
(Ask yourself…Why?)

7. Phase Transitions are Physical Changes
Keep on increasing the temperature to
100C…
The kinetic energy of the water molecules
increases and the water boils.
Some of the water molecules have enough
kinetic energy to overcome the attractive
potential (IM forces) and escape the liquid.

8. Phase Transitions and External Pressure
It is not as obvious, but the physical state of
matter depends on the pressure, as well.
The general idea…
As external pressure on a given gas increases
(at constant T), the phase changes from
gas liquid solid

9. Phase Transitions and External Pressure
Compare the 3 scenarios for gases in a piston that
were discussed in context of the van der Waals
eqn…
As the pressure on the gas is increased (at constant T),
the gas molecules are forced closer together.

10. Phase Transitions and External Pressure
The gas molecules “feel” each other’s presence
and begin to cluster (due to IM attractions).

11. Phase Transitions and External Pressure
Continue increasing the external pressure (at const. T)…
When the pressure is high enough, the gas
sequentially condenses to a liquid, then transitions
to a solid (exception- water).

12. Major Types of Phase Transitions
Melting (fusion)- solid to liquid
Evaporation (vaporization)- liquid to vapor
Sublimation- solid to vapor
Condensation- vapor to liquid
Freezing (or solidification)- liquid to solid
Deposition- vapor to solid

13. Next Up…
Vapor Pressure (Pt 2)