Not all mixtures of liquids obey Raoult’s law, such mixtures; called azeotropes, mimic the boiling behavior of
pure liquids. These mixtures when present at specific concentrations usually distill at a constant boiling
temperature and cannot be separated by distillation. Examples of such mixtures are 95% ethanol-5% water (bp
78.1 °C).
To understand the nature of simple distillation, fractional distillation and azeotropes we need to look at
vapor/liquid diagrams for pairs of solvents. The graph below (Fig. 5) shows such a diagram for 2 solvents, A
and B. A is the lower boiling material. The bottom of the graph shows the liquid state and the top of the graph
shows the vapor state. The area in between the two curves shows what is happening in the distillation column.
If we start with a mixture of A and B that corresponds to the dashed white line on the graph and the letter C1
(concentration 1). The mixture is vaporized (distilled). Follow the horizontal blue line until it reaches the vapor