Final answer:
Freezing point depression is the reduction in the freezing point of a solution versus that of the pure solvent due to the presence of a solute. This phenomenon requires a lower temperature for the solution to freeze and is proportional to the molal concentration of the solute.
Step-by-step explanation:
Freezing point depression refers to the lowering of the freezing point of a solution compared to its pure solvent. This colligative property is observable when a solute is added to a solvent, resulting in the need for a lower temperature to solidify the solution.
For example, salt spread on icy roads prevents the water from freezing at the normal freezing point of 0°C, allowing it to remain liquid down to as low as −9°C. This effect is due to the interactions between the solute and solvent molecules, which disrupt the formation of the solid phase and require more energy (or a lower temperature) to be removed for the solution to freeze.
The mathematical relationship for freezing point depression is given by ΔTf = Kfm, where ΔTf is the change in freezing point, Kf is the freezing point depression constant, and m is the molal concentration of the solute.