Final answer:
Freezing point depression lowers the freezing point while boiling point elevation raises the boiling point of a solution compared to a pure solvent. On a phase diagram, these changes shift the solid-liquid and liquid-gas curves, respectively, indicating altered phase change temperatures proportional to the solute concentration.
Step-by-step explanation:
Freezing point depression and boiling point elevation are colligative properties that describe how the addition of a solute to a solvent affects its phase change temperatures. In particular, freezing point depression occurs when the addition of a solute lowers the freezing point of a solution compared to the pure solvent. This is because the solute particles disrupt the formation of a solid crystal lattice, requiring a lower temperature to achieve the solid phase. As a result, on a phase diagram, the solid-liquid curve for a solution will intersect the line representing standard pressure (P = 1 atm) at a lower temperature than that of the pure solvent.
Boiling point elevation occurs because the addition of a solute decreases the vapor pressure of a solution. This means more heat must be applied to the solution for its vapor pressure to match the atmospheric pressure, leading to an elevated boiling point. Therefore, on the phase diagram, the liquid-gas curve will intersect the line representing standard atmospheric pressure at a higher temperature for the solution than it would for the pure solvent.
Both these effects result in a phase diagram where the lines representing the states of matter for the solution are shifted from those of the pure solvent. The magnitude of these shifts is proportional to the amount of solute added and can be calculated using specific equations for freezing point depression and boiling point elevation.