To determine the molecular weight of a compound by measuring the freezing point depression.
It is important to note that the solution process involves breaking bonds and making new bonds. The bonds between ions and molecules have to be broken, which requires energy. Some bonds within the solvent molecules have to be broken to accomodate the dissolved particles. When molecules are freed or ions are formed, energy is released out of the system. These three energy changes result in "heat of solution," which can either be positive or negative. A positive energy change suggests an endothermic reaction and a negative energy change suggests a exethermic reaction. When a solute is added to a solvent there will be less solvent molecules that can escape to the gas phase due to the bonds that are formed when they come together.
The phase diagram shows that vapor pressure over a solution is lower than the vapor pressure of the solvent at any given temperature. Since the solution will have a lower vapor pressure it will reach the atmospheric pressure at a higher temperature than the boiling point. That means that it will reach the freezing point at a lower temperature than the pure solvent. In sum, the boiling point of the solution will be higher and freezing point lower compared to the pure solvent.