In phase equilibria for vapours and liquids: A vapour whose actual partial pressure is less than its equilibrium vapour pressure at the same temperature is best described as which type of vapour?

Introduction / Context:
Engineers often classify vapours by comparing their actual partial pressure to the equilibrium vapour pressure at a given temperature. This determines whether condensation can occur and influences mass and heat transfer processes in unit operations such as distillation, drying, and humidification.

Given Data / Assumptions:

• Temperature is fixed.
• The vapour coexists with or is conceptually comparable to a flat liquid surface of the same substance.
• Equilibrium vapour pressure is the saturation pressure at that temperature.

Concept / Approach:
Definitions: If the actual vapour partial pressure p is less than the saturation pressure psat(T), the vapour is unsaturated and can accept more vapour without condensation. If p = psat, it is saturated. “Superheated” refers to a pure vapour or gas at a temperature above its saturation temperature at the same pressure, which is related but not identical to the partial-pressure definition for mixtures.

Step-by-Step Solution:

Verification / Alternative check:
Psychrometric charts show that points below the 100% relative humidity line (p/psat < 1) are unsaturated states—consistent with industrial humidity practice.

Why Other Options Are Wrong:

• Saturated: Requires p = psat(T), not less.
• Superheated: Relates to temperature above saturation at a given total pressure; not the same as p < psat for a component in a mixture.
• Dry gaseous: Non-standard term in this context and does not denote the p versus psat relationship.

Common Pitfalls:
Confusing superheat with unsaturation; they describe different departures from saturation—temperature-based versus partial-pressure-based. Also, mixing up total pressure and component partial pressure in mixtures.

Final Answer:
Unsaturated