For a gas to be liquefied by pressure, its temperature must be set in what relation to its critical temperature?
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ARaised above the critical temperature
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BLowered below the critical temperature
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CExactly at the critical temperature
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DUnrelated to the critical temperature
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ECycled above and below the critical temperature
Answer
Correct Answer: Lowered below the critical temperature
Explanation
Introduction / Context:Gas liquefaction is crucial in cryogenics, storage, and industrial processes (oxygen, nitrogen, LNG). The critical temperature sets a fundamental limit: above it, no amount of pressure alone can condense a gas into a liquid phase.
Given Data / Assumptions:
- Critical temperature T_c is a thermodynamic property of each substance.
- At T > T_c, distinct liquid and vapor phases cannot coexist (supercritical region).
Concept / Approach:To liquefy by compression, the gas must be at T < T_c so that the phase boundary exists. Then increasing pressure moves the state into the two-phase dome, producing liquid. At or above T_c, the fluid becomes supercritical and does not condense into a separate liquid phase with pressure alone.
Step-by-Step Solution:
1) Identify T_c as the maximum temperature at which liquid can exist.2) For liquefaction by pressure: ensure T < T_c.3) Apply compression to enter the saturation dome and obtain liquid.Verification / Alternative check:CO2 (T_c ≈ 31°C) demonstrates this: at room temperature above T_c, raising pressure produces a supercritical fluid rather than a distinct liquid–vapor mixture.
Why Other Options Are Wrong:
- Above or exactly at T_c: no two-phase region accessible by pressure alone.
- “Unrelated” or “cycled” statements contradict phase-equilibrium fundamentals.
Common Pitfalls:Confusing supercritical fluid density changes with true condensation; visible meniscus and latent heat vanish above T_c.
Final Answer:Lowered below the critical temperature