A gas can be liquefied only if it is cooled below its
Correct answer: B. critical temperature
- A. boiling point
- B. critical temperature
- C. melting point
- D. triple point
Explanation
Above the critical temperature no amount of pressure will produce a liquid, because the molecules have too much kinetic energy for the attractive forces to hold them together. This is why oxygen and nitrogen must be cooled deeply before they can be liquefied while ammonia liquefies at room temperature under pressure. The pressure needed at that temperature is the critical pressure.
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About Real and Ideal Gases
An ideal gas is modelled as particles with negligible volume and no intermolecular attraction, obeying Boyle's, Charles's and Avogadro's laws through PV = nRT. Real gases deviate from this behaviour, especially at high pressure and low temperature, because particle volume and attractive forces matter; the van der Waals equation explains these corrections.
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