1.3.3. Depression of Freezing Point

The lowering of vapour pressure of a solution causes a lowering of the freezing point compared to that of the pure solvent. We know that at the freezing point of a substance, the solid phase is in dynamic equilibrium with the liquid phase. Thus, the freezing point of a substance may be defined as the temperature at which the vapour pressure of the substance in its liquid phase is equal to its vapour pressure in the solid phase. A solution will freeze when its vapour pressure equals the vapour pressure of the pure solid solvent.

When we add a non-volatile solute to a solvent, it decreases the solvent's vapour pressure. This means that the temperature at which the solvent can solidify (freeze) is also lowered. Essentially, a solution freezes at a lower temperature than the pure solvent. This change occurs because the presence of solute particles hinders the ability of the solvent molecules to escape into the vapour phase, which in turn means it takes a lower temperature for the vapour pressures of solid and liquid phases to balance and reach equilibrium.

Think of this concept in terms of a freezing lake in winter. If you throw salt (the solute) onto the ice, it prevents the water underneath from freezing solidly, allowing some liquid water to remain. This is similar to how adding salt lowers the freezing point of water, preventing ice from forming under normal conditions.

Let T0 be the freezing point of pure solvent and Tf be its freezing point when non-volatile solute is dissolved in it. The decrease in freezing point ΔT = T0 − Tf is known as the depression in freezing point. Similar to elevation of boiling point, depression of freezing point (ΔTf) for dilute solution (ideal solution) is directly proportional to molality, m of the solution. Thus, ΔTf ∝ m or ΔTf = Kf × m.

The mathematical relation shows that the depression in freezing point is proportional to the molality of the solution. The constant Kf, known as the freezing point depression constant, is specific to the solvent. In simple terms, the more solute you add to a solvent (in terms of quantity relevant to the solvent's mass), the lower the freezing point will drop. By using this relationship, we can calculate how much the freezing point of a solvent will change when specific amounts of solute are added.

Consider how road salt lowers the freezing point of water on icy roads. When enough salt is applied, the freezing point of the water is lowered to below the ambient temperature, which prevents ice from forming. The effectiveness of salt in this context can be predicted using the freezing point depression formula.

The proportionality constant, Kf, which depends on the nature of the solvent is known as Freezing Point Depression Constant or Molal Depression Constant. The unit of K is K kg mol-1. Values of K for some common solvents are listed in table form for practical reference.

The freezing point depression constant is critical in calculations involving different solvents. For example, different solvents will have different values for Kf, affecting how much a solute can lower the freezing point. These values allow chemists to predict the behavior of solutions under various conditions, like in antifreeze formulations where knowing how to precisely control freezing points is vital.

Imagine antifreeze added to your car's radiator. The Kf values for the solutions determine how effective the antifreeze is at keeping the engine from freezing in cold weather. By selecting the right solvents and solutes based on their Kf values, the right concentration can be achieved to ensure the vehicle remains operable in low temperatures.

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