Azeotropy and Raoult’s law: a binary azeotrope that boils at a temperature lower than either pure component exhibits which type of deviation from Raoult’s law?

Introduction / Context:
Azeotropes are constant-boiling mixtures that behave non-ideally relative to Raoult’s law. Whether an azeotrope boils lower or higher than its pure components depends on the sign of deviation from ideal solution behavior, which is crucial for separation strategy decisions.

Given Data / Assumptions:

• Binary mixture forms an azeotrope.
• Azeotropic boiling point is lower than that of either pure component at the same pressure.
• Raoult’s law describes ideal solutions; deviations reflect intermolecular interaction changes on mixing.

Concept / Approach:
Positive deviation means that component–component interactions are weaker than like–like interactions; the total vapor pressure is higher than predicted by Raoult’s law, leading to a lower boiling temperature. Hence, a minimum-boiling azeotrope corresponds to positive deviation. Negative deviation yields a maximum-boiling azeotrope (higher boiling than either component).

Step-by-Step Solution:

Verification / Alternative check:
Phase diagrams for ethanol–water (minimum-boiling) exemplify positive deviation from Raoult’s law.

Why Other Options Are Wrong:

• Negative deviation corresponds to maximum-boiling azeotropes.
• No deviation is the ideal case, which does not form azeotropes.
• “Cannot be predicted” is incorrect given the information.

Common Pitfalls:
Confusing minimum- and maximum-boiling azeotropes; assuming all azeotropes have the same deviation sign.

Final Answer:
Positive deviation