Non-ideality in liquids — The activity coefficient γ of a component in a liquid solution accounts for departure from ideal solution behavior. Which statement is true?

Introduction / Context:
Real liquid mixtures deviate from Raoult’s-law ideality due to molecular size, shape, and intermolecular forces. The activity coefficient γ modifies the effective concentration (activity) to capture these deviations. Understanding what γ depends on, and what it does not directly represent, is essential for vapor–liquid equilibrium (VLE) and separation process design.

Given Data / Assumptions:

• Activity a_i = γ_i * x_i for liquid-phase component i (at moderate pressures).
• γ_i depends on composition and temperature (and sometimes pressure weakly).
• Colligative properties are separate phenomena emergent from solution behavior, not direct definitions of γ.

Concept / Approach:

In ideal solutions, γ_i = 1. In non-ideal solutions, γ_i ≠ 1 and varies with the mixture composition and temperature because local molecular environments change with composition and thermal energy. While boiling point elevation or freezing point depression can be predicted using activities, γ itself is not “the measure” of those properties; rather, such properties emerge from the full thermodynamic relations.

Step-by-Step Solution:

Verification / Alternative check:

Models such as Wilson, NRTL, UNIQUAC compute γ_i as functions of composition and temperature, fitting binary parameters to data.

Why Other Options Are Wrong:

A/D: Boiling point elevation and freezing point depression are colligative outcomes, not definitions of γ. B: γ typically changes with T. E: γ = 1 only for ideal behavior; many mixtures are non-ideal.

Common Pitfalls:

Using constant γ over wide composition ranges; ignoring temperature dependence in design calculations.

Final Answer:

It is a function of the liquid-phase composition