The activity described here forces students to reconcile the
differences between a liquidvapor system of a single substance
versus a system that only includes a gas. Although this is a subtle
difference to students, altering the volume of the systems results
in totally different sets of behaviors. Thus, students must
conclude that the liquidvapor system maintains constant
pressure due to phase equilibrium yet the system with only a
gas is dominated by Boyle’s law. Students must realize the rate
of vaporization changes only with temperature; however, the
rate of condensation adjusts with volume. When the volume is
reduced, the frequency of gas molecules colliding with the
liquid surface increases, which increases the rate of condensation.
This increase is only temporary, and then the rate of
condensation stabilizes for the new volume to again equal the
rate of vaporization and reestablish equilibrium. Through this
line of reasoning, the idea of dynamic equilibrium is illustrated
and students come to see that particles are in constant movement
within a systemeven though it appears stationary from the
macroscopic view.