This document discusses vapor pressure and how it relates to phase transitions of substances. It defines vapor pressure as the pressure created by molecules escaping from the liquid phase due to sufficient kinetic energy. The vapor pressure is at equilibrium when the rates of evaporation and condensation are equal. Vapor pressure increases with temperature because molecules have more energy to escape the liquid. The boiling point of a liquid is reached when the vapor pressure equals the external pressure, usually 1 atmosphere.

1. Vapor Pressure
(Phase Transitions Pt. 2)
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. Vapor Pressure
All liquids (or solids) continuously
“vaporize”.
Think about what happens to a glass of
water if you leave it out over several
days… it evaporates.

3. Vapor Pressure
The vapor pressure of a substance is
The pressure created over a liquid by
the molecules having enough kinetic
energy to escape the liquid.
Thermodynamically, it is the partial
pressure of a gas at equilibrium.

4. Equilibrium Vapor Pressure
Molecules in the liquid phase
continuously vaporize and
condense in a closed container.
rate of evaporation = rate condensation
The partial pressure of the gas is
constant at equilibrium.
Liquid molecule

5. Equilibrium Vapor Pressure
The equilibrium vapor pressure for a
substance is higher for substances with
weak intermolecular forces.
Volatile liquids evaporate “quickly”
Nonvolatile liquids have strong
intermolecular attractions and evaporate
more slowly.

6. Vapor Pressure and Temperature
The vapor pressure for a substance
increases with increasing temperature.
(Ask yourself…Why?)
When the external pressure equals the
vapor pressure for a given liquid, it boils.

7. Vapor Pressure and Temperature
The boiling point is the temperature
where the vapor pressure equals the
external (usually atmospheric) pressure.
Boiling point
vapor pressure = external pressure (usually 1 atm)

8. Vapor Pressure and Temperature
The vapor pressure produced by a liquid
increases with increasing T.
Temperature
Vapor
Pressure
“boiling line” or
liquid-vapor
coexistence line

9. Vapor Pressure and Temperature
A plot of the vapor pressure as a
function of temperature
Temperature
Vapor
Pressure
TB
Pext = VP
Liquid-vapor
coexistence line
gives the vapor
pressure of a
substance at a
given T

10. Next Up…
Phase Diagrams (Pt 3)