Azeotropes and Raoult’s law deviations:\nAn azeotropic mixture of two liquids exhibits a boiling point that is lower than the normal boiling point of either pure component when the solution shows which type of deviation from Raoult’s law?

Introduction / Context:
Azeotropes are special liquid mixtures that boil at a constant temperature and composition. Distillation cannot separate their components beyond the azeotropic composition under the same pressure. Understanding how deviations from Raoult’s law (positive or negative) determine whether an azeotrope has a minimum or maximum boiling point is widely tested in chemical engineering thermodynamics and separation process design.

Given Data / Assumptions:

• Binary, nonreactive liquid mixture.
• Equilibrium at a fixed total pressure (commonly 1 atm).
• Raoult’s law serves as the ideal-solution reference: p_i = x_i * p_i^sat.
• Positive deviation means weaker unlike-molecule attractions; negative deviation means stronger unlike interactions.

Concept / Approach:
When a mixture shows positive deviation from Raoult’s law, the escaping tendency (fugacity) of each component is higher than ideal. This raises the total vapor pressure above the ideal-solution prediction at a given temperature. Since boiling at fixed pressure occurs when total vapor pressure equals the external pressure, a higher total vapor pressure at the same composition implies the mixture reaches the boiling condition at a lower temperature than either pure component—hence a minimum-boiling (low-boiling) azeotrope. Conversely, negative deviation lowers total vapor pressure, yielding a maximum-boiling azeotrope.

Step-by-Step Solution:
Define deviation: positive deviation → p_total(actual) > p_total(ideal).At fixed P, higher p_total at a given T means the mixture will boil at a lower T than predicted for ideal behavior.If the boiling point falls below both pure-component boiling points at the azeotropic composition, a minimum-boiling azeotrope forms.Therefore, “lower than either” corresponds to positive deviation.

Verification / Alternative check:
Classical examples: ethanol–water (minimum-boiling azeotrope at ~95.6% ethanol by mass at 1 atm) exhibits positive deviation; hydrochloric acid–water (maximum-boiling) exhibits negative deviation.

Why Other Options Are Wrong:
Is saturated / Is unsaturated: these do not describe deviations from Raoult’s law and do not determine azeotrope type.Negative deviation: associated with maximum-boiling azeotropes (boiling point higher than either component).

Common Pitfalls:
Confusing minimum- and maximum-boiling azeotropes; assuming azeotropes always boil lower; ignoring pressure effects (changing pressure can create or eliminate azeotropes).

Final Answer:
shows positive deviation from Raoult's law