Vapor states and definitions: Identify the incorrect statement among standard definitions of saturated and superheated vapor.

Introduction / Context:Understanding precise thermodynamic terminology is critical in phase-equilibrium calculations and equipment design. This item checks your familiarity with saturated vapor, superheated vapor, boiling point, and dew point, and asks you to spot a subtle but important misuse of terms.

Given Data / Assumptions:

• Standard definitions of saturation, superheat, boiling point, and dew point are used.
• Pure-substance behavior is implied unless otherwise specified.

Concept / Approach:A saturated vapor exists at its equilibrium vapor pressure for the given temperature (or equivalently at its saturation temperature for the given pressure). Superheat is the temperature above saturation at the same pressure. Dew point is the temperature at which a gas mixture just begins to condense at a given pressure; it is a mixture concept, not the definition of pure-substance saturation temperature.

Step-by-Step Solution:

Verification / Alternative check:For pure water at 1 atm, saturation temperature is 100 °C. If steam at 1 atm is at 120 °C, degree of superheat is 20 °C. The “dew point” is used for air–water vapor mixtures; using it for pure steam is not standard.

Why Other Options Are Wrong:

• (a), (b), and (c) are standard, accepted definitions.
• (e) is invalid because (d) is not fully correct.

Common Pitfalls:Confusing dew point (mixture property) with saturation temperature of a pure component; mixing pressure- and temperature-specified states; and forgetting the “same pressure” condition when defining superheat.

Final Answer:The degree of superheat equals the difference between the vapor’s temperature and its saturation temperature (i.e., dew point).