Phase Equilibrium in Ideal Mixtures

Abstract

In this chapter, we use the ideal mixture model to phase transitions and related phenomena that occur in two-component systems. In the context of liquid–gas equilibrium, we discuss Raoult’s and Henry’s laws. According to Raoult’s law the partial vapour pressure of a component above an ideal solution is proportional to the molar fraction of that component in the solution. Henry’s law is similar to Raoult’s law but applies only to dilute solutions, for instance, to a small amount of a gaseous component dissolved in a liquid solvent, in equilibrium with the gaseous phase. Then we discuss the liquid–solid equilibrium, in particular, solubility of solids in liquids and the phase diagram of a system called the simple eutectic. We also discuss osmotic equilibrium which occurs between a pure solvent and the solvent in a solution in the presence of a membrane permeable only to solvent molecules. Finally, we summarize the properties of solutions which depend only on the amount of the solute but not on its characteristics, called colligative properties. Examples of colligative properties are the boiling point elevation, freezing point depression and osmotic pressure.