Final answer:
The average kinetic energy of the remaining molecules decreases when high-energy molecules are lost in vaporization. The molecules with higher kinetic energy escape the liquid, leading to a lower average kinetic energy among the molecules left in the liquid phase.
Step-by-step explanation:
When high-energy molecules are lost in vaporization, the average kinetic energy (KEavg) of the remaining molecules decreases. This is because during vaporization, the molecules with higher kinetic energy escape from the liquid into the gas phase. As a result, the remaining liquid has a lower average kinetic energy. This would make the correct answer to the student's question A) Decreases.
Conversely, if we were to increase the temperature of a gas or liquid, we see an increase in KEavg, meaning more molecules have higher speeds. The energy distribution shifts toward higher speeds, as can be demonstrated with nitrogen gas as per Figure 9.33. In the context of evaporation and condensation, it's important to note that energy is used to break bonds between particles during phase changes, which doesn't necessarily increase the kinetic energy of the system's particles (as seen in vaporization), but instead, the potential energy changes.