Thermodynamic terminology: a vapor existing above its critical temperature is properly called a what?

Introduction / Context:
Correct phase terminology is crucial in phase-equilibrium calculations and equipment selection. Above the critical temperature, a substance cannot be liquefied by pressure alone, which changes how we refer to the phase.

Given Data / Assumptions:

• Temperature is above the critical temperature of the substance.
• Standard definitions from thermodynamics apply.

Concept / Approach:
For T > T_c, there is no distinct liquid–vapor phase transition upon compression; the substance behaves as a gas. The term “vapor” is commonly reserved for T below T_c where condensation by pressure is possible. “Supercritical fluid” refers to conditions above both the critical temperature and critical pressure and is not one of the provided canonical options.

Step-by-Step Solution:
Recognize that at T > T_c, liquefaction by pressure is impossible.Therefore, the appropriate term is “gas” (gaseous state), not “vapor.”Select the option labeled “gaseous.”

Verification / Alternative check:
Phase diagrams show the vapor–liquid dome ending at the critical point; to the right (higher T) the region is simply gas unless at high pressure where the term supercritical fluid applies.

Why Other Options Are Wrong:
“Saturated”/“sub-cooled” describe near-boiling or below-saturation conditions; “unsaturated” is nonstandard here; “supercritical vapor” is imprecise terminology.

Common Pitfalls:
Using “vapor” and “gas” interchangeably without regard to T_c; calling everything above T_c “supercritical” regardless of pressure.

Final Answer:
gaseous