Nonideal liquid solutions, also known as real solutions, do not strictly follow Raoult's law. Raoult's law is a rule of thumb in physical chemistry. However, not all mixtures adhere to this law due to varying molecular interactions. For example, in an acetone/chloroform solution, the individual vapor pressures of the components are lower than expected, resulting in a total vapor pressure below that predicted by Raoult's law, causing a negative deviation.
On the other hand, in an ethanol/water solution, each component has a higher vapor pressure than expected, leading to an increase in total pressure. This is referred to as a positive deviation from Raoult's law.
The temperature-composition phase diagram of a system with positive deviation displays unique characteristics. The bubble point line, which is a line representing the temperature at which the first bubble of vapor forms during heating, and the dew point line, which represents the temperature at which the vapor starts to condense, intersect at a specific point before diverging. At this intersection, the liquid and vapor in equilibrium with the liquid have the same mole fraction, and the system behaves like a pure component. This specific composition is called the azeotropic composition, and the pure component sharing this composition is termed an azeotrope.
In systems showing a positive deviation, the azeotrope has the lowest boiling point, making it a minimum-boiling azeotrope. An azeotropic mixture of 96% ethanol and 4% water is an example of a minimum-boiling azeotrope that boils at a temperature of 78.2°C.
Conversely, for systems with a negative deviation, the intersection of the bubble point and dew point lines indicates the maximum-boiling azeotrope.
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.
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