FREEZING-POINT LOWERING THOMAS B. GREENSLADE The City CdlIege of the College of the City of New York
T
HE lowering of the freezing point of a pure solvent by the addition of a solute is a common occurrence in everyday life. Yet few texts in firstyear college chemistry attempt an explanation of this phenomenon. Some authors "explain" the lowering by means of a diagram showing the vapor pressure-temperature relationships between ice, water, vapor, and solution. It must be pointed out, however, that this method of approach is not understood by first-year students, and does not effect an explanation of the phenomenon, but rather a proof that equilibrium may exist only a t the lowered temperature. Since almost all texts explain the lowering of the vapor pressure of a solvent and the elevation of the boiling point due to the addition of a non-volatile solute on the basis of the kinetic molecular theory, it would seem desirable to undertake an analogous explanation for the freezing-point depression. With this idea in mind, the following explanation is offered for use in first-year college classes. Molecules in the solid state are in constant vibratory motion. The kinetic energy of these molecules increases as the temperature increases. Some of those near the surface may gain sufficient energy to overcome the forces holding them in place in the crystal lattice.
In that case they break away from the solid and enter the liquid state. As soon as any liquid is formed, some of the molecules in the I i w d state begin to go back to the solid state. When the number of molecules leaving the solid and going into the liquid state is equal to the number going from the liquid to the solid, equilibrium is established. Let us consider a unit surface between water and ice in equilibrium at O°C. In any given unit of time there are as many molecules leaving the ice and entering the liquid state as there are molecules leaving the water and forming solid. Suppose now that we add a solute such as sodium chloride to the mixture. The salt dissolves in the water, and we have a solution in contact with ice. Since the number of water molecules per unit volume in the liquid has been diminished by this addition of solute molecules, the rate of ice formation is decreased. Now there are more molecules changing from the solid to the liquid state than are returning to the solid, heat is absorbed, and the temperature falls. This lowering of the temperature continues until it effects an equalization of the rates of ice and water formation. At the temperature a t which this occurs a new equilibrium is established between ice and solution, and they can exist in contact with each other. This is, by definition, the freezing point of the solution.
