Critical temperature (thermodynamics) Select the correct statement that defines the critical temperature of a substance.
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ABelow it, a gas does not obey gas laws
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BAbove it, a gas may explode
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CBelow it, a gas is always liquefied
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DAbove it, a gas will never be liquefied by pressure alone
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EIt is the temperature of maximum density of a gas
Answer
Correct Answer: Above it, a gas will never be liquefied by pressure alone
Explanation
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