When a volatile liquid is stored in an open container the liquid eventually evaporates, leaving the container
empty. In contrast, if a liquid is stored in a sealed container, the liquid will evaporate while the vapor
re-condenses simultaneously. The equilibrium vapor pressure is the pressure exerted by the vapor when equilibrium
between vaporization and condensation is reached. It is a measure of the volatility of a compound, i.e. the tendency
of its molecules to escape from the liquid phase and enter the vapor phase at a given temperature. When equilibrium
between these two states occurs, a state of dynamic equilibrium is established. Note that "vapor pressure,"
albeit incorrect, is the common name for the equilibrium vapor pressure. This module explores the effects of temperature
and molecular size and shape on the vapor pressure of liquids. All liquids have some fraction of molecules
that have sufficient energy to break their attraction to other molecules and escape into the vapor phase.
The ability to do this depends on both the strength of those attractions and the temperature of the liquid.
INSTRUCTIONS
The vapor pressure of each liquid is reported underneath the beakers in units of mmHg. In addition, the
relative amount of compound in the gas phase is displayed pictorially as dots in the flask. The number of
dots shown is proportional to the number of molecules in the gas phase, and therefore the vapor pressure.
A plot of the pressure as a function of temperature for the two liquids is also representative of the conditions
for equilibrium of liquid and its vapor. Note that the pressure on the Y-axis of the plot has a factor of 102.
Last modified: April 13, 2000