Raoult's law: under which condition does the proportionality between a component's vapor pressure and its liquid-phase mole fraction fail to apply?

Introduction / Context:
Raoult's law describes ideal solutions where each component's partial vapor pressure equals its mole fraction times the pure-component vapor pressure. Ideal behavior requires similar intermolecular interactions between like and unlike species. Many real mixtures deviate from this ideality, especially when specific interactions or reactions occur upon mixing. Knowing when Raoult's law breaks down is essential for accurate phase-equilibrium calculations and for selecting activity-coefficient models.

Given Data / Assumptions:

• Temperature and pressure are moderate so vapor behaves nearly ideally.
• We are evaluating liquid-phase nonideality conditions.
• Key assumptions for Raoult's law include comparable molecular size, nonpolarity, and no preferential surface enrichment.

Concept / Approach:
When unlike molecules interact much more strongly or weakly than like molecules, or when chemical association or reaction occurs, the solution is nonideal. Chemical association between unlike molecules (e.g., hydrogen bonding, complex formation, acid–base reactions) alters escaping tendencies, so vapor pressures are not proportional to simple mole fractions. In such cases, activity coefficients depart from unity, and Raoult's law must be corrected using models like Margules, Wilson, NRTL, or UNIQUAC.

Step-by-Step Solution:
Recall Raoult's law requirement: similar interactions and no reaction on mixing.Identify condition that violates this: chemical combination or association between unlike molecules.Conclude Raoult's law does not apply under that condition.Select the option describing molecular association/chemical combination.

Verification / Alternative check:
Classical examples include acetone–chloroform (specific H-bonding) and acid–base pairs, all showing strong deviations from Raoult's law and requiring activity coefficients substantially different from 1.

Why Other Options Are Wrong:
No surface concentration, similar size, and comparable attractions are exactly the ideal-solution assumptions under which Raoult's law is valid.

Common Pitfalls:
Assuming any nonideality invalidates Raoult's law entirely; mild nonideality can be handled by activity coefficients, but strong association requires more advanced models.

Final Answer:
There is chemical combination or molecular association between unlike molecules on mixing.