Critical temperature (thermodynamics) Select the correct statement that defines the critical temperature of a substance.

Introduction / Context:
Critical properties are pivotal for refrigeration, power cycles, and supercritical fluids. The critical temperature establishes the boundary between subcritical condensation and supercritical behavior.

Given Data / Assumptions:

• Pure substance; equilibrium thermodynamics.
• Critical point corresponds to Tc, pc, and critical specific volume.

Concept / Approach:
At temperatures above the critical temperature Tc, distinct liquid and vapor phases cease to exist; compression alone cannot induce liquefaction. Below Tc, sufficient pressure can condense the vapor.

Step-by-Step Solution:
Identify Tc as the upper limit for phase coexistence.Interpretation: for T > Tc, only a supercritical fluid exists; applying pressure does not cross a phase boundary.Therefore, the correct definition is that above Tc, a gas cannot be liquefied by pressure alone.

Verification / Alternative check:
Examine CO₂: Tc ≈ 31 °C. At 40 °C, even very high pressures do not yield a liquid phase—consistent with the definition.

Why Other Options Are Wrong:

• Obedience to gas laws is unrelated; “always liquefied below Tc” is false without sufficient pressure; explosion is not part of the definition; maximum density statement is unrelated.

Common Pitfalls:
Confusing “cannot liquefy above Tc” with “cannot compress”—compression still increases density but without phase change.

Final Answer:
Above it, a gas will never be liquefied by pressure alone