Question

When some liquid evaporates, the average speed of the molecules remaining will ____.
(A) Increase because the more energetic molecules have left
(B) Decrease because the more energetic molecules have left
(C) Remain unchanged because all molecules have about the same speed
(D) Increase because there are fewer molecules

Answer 1

Hint: While solving this question, we need to know that the increased speed of the liquid molecules is a direct consequence of the increase in energy of the molecules. This energy is specifically known as kinetic energy.

Complete answer:
Evaporation is a type of vaporization in which the liquid phase transitions into gaseous phase. It is a surface phenomenon, which means that only the liquid particles at the surface take part in this process. For phase transition or conversion of matter from one phase to another, energy must be gained or lost. In evaporation where the transition is occurring from liquid phase to gaseous phase, energy must be gained by the liquid molecules in order to break free of their attractive forces and overcome their potential energy.
When a liquid is heated, the molecules gain kinetic energy and their movement is rapid. During this process, the molecules bump into each other, imparting their energy to other molecules which have increased speed now as well. The molecules at the surface, on gaining sufficient energy, escape into the atmosphere as vapour phase, leaving behind those molecules which have not gained enough energy, and hence have lower speeds. Therefore, when some liquid evaporates, the average speed of the molecules remaining will (B) decrease because the more energetic molecules have left.

Note:
The average kinetic energy of the liquid molecules is proportional to the temperature of the liquid, meaning at higher temperatures, more energy transfer occurs, increasing kinetic energy and thereby average speed of the molecules, causing them to leave the liquid surface and transition into gas. This means that the average speed of the liquid molecules is also dependent on the temperature of the liquid.