Spooky Chemistry: The Science of Freezing Point Depression

What is cryoscopic descent? It is the decrease in the temperature of the freezing point that a solution experiences with respect to that of the pure solvent, all solutions have a lower freezing temperature than the pure solvent.

The lowering of the freezing temperature of a solvent due to the presence of a solute is used to prevent the solidification of cooling water in combustion engines. In cold regions, where the temperature can fall below 0 °C, substances are added to the cooling water to lower its freezing temperature and thus prevent it from freezing, since, if it occurs, the increase in the volume of the ice could break the cooling system.

The cryoscopic descent can be explained from the change in entropy (physical magnitude for a thermodynamic system in equilibrium) that occurs during the phase change. Thus, a pure solid is more ordered than a pure liquid, and therefore the latter has a greater entropy, a greater disorder. The disorder is due to the fact that the particles (molecules, atoms or ions) of a solid occupy a fixed position and only vibrate around that position.

On the contrary, in a liquid the particles are in motion and do not have a determined position, a liquid solution has more disorder than a pure liquid since in the solution, in addition to the particles of the solvent in motion, there are also particles of moving solute, making the system more disordered.

It’s time to get active! Let’s carry out the following practice with great caution, and see the cryoscopic descent in action.