Question

A gas can be liquefied ______.

• A. above its critical temperature
• B. at its critical temperature
• C. below its critical temperature
• D. at any temperature

Answer 1

Answer: (C)

below its critical temperature

A gas can be liquefied below its critical temperature.

For example, the critical temperature $T_c$ for $C{O}_2$, $O_2$ and $H_2$ are 31.1 ${}^{0}C$, $-$118.8 ${}^{0}C$ and $-$240 ${}^{0}C$ respectively.

Thus, $C{O}_2$ gas cannot be liquefied above $31.1{}^{0}C$ however high the pressure may be applied.