Nonideal Two-Component Liquid Solutions

Nonideal liquid solutions deviate from Raoult's law because of differences in molecular interactions. Strong solute–solvent interactions lower vapor pressure, causing a negative deviation, while weak interactions increase vapor pressure, resulting in a positive deviation.

For example, the acetone-chloroform solution has lower individual vapor pressures than those predicted by Raoult's law, resulting in a lower total pressure and a negative deviation.

However, an ethanol-water solution shows higher individual vapor pressures than expected, increasing the total pressure and causing a positive deviation.

The temperature-composition phase diagram for a nonideal solution shows an intersection of the bubble and dew point lines, representing the azeotropic composition.

At this point, the liquid and vapor phases in equilibrium have identical mole fractions, behaving as an azeotropic pure component.

As azeotropes boil at a constant temperature, they are called constant boiling solutions.

In a system with positive deviation, the azeotrope appears at the lowest boiling temperature, forming a minimum-boiling azeotrope, whereas with negative deviation, the intersection gives a maximum-boiling azeotrope.