A molecular motion-based approach to entropy and application to phase transitions and colligative properties

Freezing point depression and boiling point elevation are colligative properties that are taught in many undergraduate science curricula, often by a discussion of the change in chemical potential of the solution, or by referring to interactions between solute and solvent molecules, which evades the major entropy-driven effect. In this teaching proposal, we suggest introducing thermodynamics by a simplified and visual statistical method based on Maxwell-Boltzmann distribution plots that can be related to entropy. This approach allows for an entropy-based explanation of phase transition temperatures, freezing point depression and boiling point elevation. It focuses on showing that these colligative properties, in the limit of ideal systems, are caused exclusively by the increased number of microstates of the solution compared to the pure solvent. The disorder metaphor, which is often used to make the entropy concept more concrete, may be useful to discuss some aspects of this phenomenon. The statistical approach, however, is a more rigorous way to explain the links between molecular motion, entropy, and colligative properties.