Nonideal Solutions: Nonideal solutions are mixtures that do not follow Raoult’s law, which means that the partial vapour pressure of each component in the solution is not directly proportional to its mole fraction in the solution. 

Types of Nonideal Solutions

Nonideal solutions are typically characterized by two types of deviations from Raoult’s law: positive deviations and negative deviations.

• Positive deviation: A positive deviation from Raoult’s law occurs when the partial vapour pressure of each component in the solution is greater than what would be expected based on Raoult’s law. This happens when the intermolecular forces between the components are stronger than those between the same component molecules. As a result, the molecules of the components prefer to stay close to each other, which reduces the number of molecules available to escape into the vapour phase, leading to a higher-than-expected partial vapour pressure. Examples of positive deviation include the mixture of ethanol and water.
• Negative deviation: A negative deviation from Raoult’s law occurs when the partial vapour pressure of each component in the solution is less than what would be expected based on Raoult’s law. This happens when the intermolecular forces between the components are weaker than those between the same component molecules. As a result, the molecules of the components prefer to stay away from each other, which increases the number of molecules available to escape into the vapour phase, leading to a lower-than-expected partial vapour pressure. Examples of negative deviation include the mixture of hydrogen chloride and water.

Properties of Nonideal Solutions

Nonideal solutions have several properties that differ from those of ideal solutions. Some of these properties are:

• Deviations from Raoult’s law: Nonideal solutions deviate from Raoult’s law, which states that the partial vapour pressure of each component in the solution is proportional to its mole fraction in the solution. Deviations can be positive or negative, depending on the nature of the intermolecular forces between the components.
• Changes in volume: Nonideal solutions can undergo changes in volume when the components are mixed together. This can happen because of differences in the size of the molecules or differences in the shape of the molecules.
• Changes in enthalpy: Nonideal solutions can undergo changes in enthalpy when the components are mixed together. This can happen because of differences in the intermolecular forces between the components.
• Changes in entropy: Nonideal solutions can undergo changes in entropy when the components are mixed together. This can happen because of differences in the disorder of the molecules in the solution.
• Nonlinear relationship between concentration and property: The relationship between the concentration of a nonideal solution and its properties (such as vapour pressure, boiling point, or freezing point) is often nonlinear. This is because the interactions between the molecules in the solution are complex and cannot be easily described by simple equations.

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By Team Learning Mantras