Final answer:
When a solute is dissolved in a solvent, it disrupts the process of ordering the solvent particles during freezing, causing the freezing point of the solution to be lower than that of the pure solvent. The magnitude of the freezing point depression is directly proportional to the molality of the solution.
Step-by-step explanation:
When a pure solvent freezes, its particles become more ordered as the intermolecular forces that operate between the molecules become permanent. In the case of water, the hydrogen bonds make the hexagonally-shaped network of molecules that characterizes the structure of ice. By dissolving a solute into the liquid solvent, this ordering process is disrupted. As a result, more energy must be removed from the solution in order to freeze it, and the freezing point of the solution is lower than that of the pure solvent. The magnitude of the freezing point depression is directly proportional to the molality of the solution. The equation is:
ΔTf = Kf×m
The proportionality constant, Kf, is called the molal freezing-point depression constant. It is a constant that is equal to the change in the freezing point for a 1-molal solution of a nonvolatile molecular solute. For water, the value of Kf is -1.86°C/m. So, the freezing temperature of a 1-molal aqueous solution of any nonvolatile molecular solute is -1.86°C. Every solvent has a unique molal freezing-point depression constant. These are shown in the table below, along with a related value for the boiling point called K.