0.17 Phase equilibrium in solutions  (Page 7/9)

We can measure the freezing point of solutions of various concentrations, and similar to the change in vapor pressure, the experimental data show that the lowering of the freezing point is proportional to the mole fraction of the solute in the liquid phase. This leads us to consider how the presence of the solute disrupts the dynamic equilibrium and produces the change in the freezing point.

To develop a model to understand this, we need another experimental observation. When frozen water is in equilibrium with a salt solution, the solid water is pure water, containing no salt. As you may have read, icebergs floating in the ocean are actually pure water, not frozen salt water. When the salt water from the ocean freezes, the salt remains behind in unfrozen water, producing ice which is pure water without any salt. You can run this experiment yourself by freezing a sample of salt water, removing the ice on the top, and then melting that ice. The water produced is pure, not a salt solution.

This observation raises two questions: why is the solid ice made only of pure water instead of a frozen salt solution? And how does this disrupt the dynamic equilibrium between the liquid and the solid? Let’s focus first on developing a model that helps answer the second question. For a pure liquid in equilibrium with a pure solid, e.g. ice and water at 0 ºC and 1 atm pressure, the rate of freezing and the rate of melting must be equal. The rate of melting depends on the fraction of the molecules in the solid that have sufficient energy to break free of the crystalline structure of the solid, which depends on the temperature. At higher temperatures, more molecules have sufficient energy, and the solid melts. This is why liquids exist at temperatures higher than temperatures where solids exist. The rate of freezing depends on how frequently molecules in the liquid collide with the surface of the solid and are deposited there, losing their energy to the surrounding liquid molecules. This depends on the temperature, but also depends weakly on the pressure applied to the liquid and solid equilibrium.

Consider now adding the solute to the liquid solvent. Since the solute does not enter the solid phase, the rate at which molecules can escape the solid and join the liquid phase does not depend on the presence of the solute. So, the rate of melting is unchanged by the presence of the solute. But we know that the freezing point is lowered by the presence of the solute in the liquid phase. Since the dynamic equilibrium is disrupted by the solute, it must be true that the rate at which molecules enter the solid phase is changed by the presence of the solute. If we don’t change the temperature, adding the solute to the solid-liquid equilibrium causes the solid to melt. So it must be true that, when the solute is present, the rate of melting at the freezing point is greater than the rate of freezing. This means that the rate of freezing of molecules onto the surface of the solid is reduced when the solute is present.