When a solute is added to a solvent it lowers the chemical potential, i.e. the free energy. This means that the liquid form of a substance will have a lower chemical potential than the pure liquid, which will change its phase behavior for both freezing and boiling. The effect on boiling is easiest to see since we can immediately envision the lowering of the vapor pressure of boiling water is salt is added to it. This means that the water would stop boiling at its normal boiling point of 373 K and we would need to increase its temperature a few degrees for it to start boiling once again. THe freezing point effect is a bit more difficult to see. We must imagine the pressure effect on ice. If we press on ice it will tend to melt. We can call this the ice skate effect and it is related to the Clapeyron equation. Thus, if ice and water are in equilibrium and one adds a solute, which lowers the vapor pressure, the tendency of the atmospheric pressure will be to cause the ice to melt. One would need to lower the temperature again to reestablish equilibrium. Hence, the freezing point is depressed.
