Q.Gases possess characteristic critical temperature which depends upon the magnitude of intermolecular forces between the gas particles. Critical temperatures of ammonia and carbon dioxide are 405.5 K and 304.10 K respectively. Which of these gases will liquify first when you start cooling from 500 K to their critical temperature ?

Step 1 – Recall the meaning of critical temperature ($T_c$)

$T_c$ is the temperature above which a gas cannot be liquefied no matter how much pressure is applied. A gas liquefies once it is cooled to (and below) its $T_c$, provided sufficient pressure is applied.

Step 2 – Compare the given data

Step 3 – Track the cooling process from 500 K

As the temperature is lowered from 500 K:

Ammonia's critical temperature (405.5 K) is reached before carbon dioxide's (304.10 K) as the system cools.

Step 4 – Conclusion …

Compare critical temperatures directly: on cooling from a temperature above both, the gas with the HIGHER $T_c$ is encountered first and liquefies …

• Assuming the gas with the LOWER $T_c$ liquefies first — it's the opposite: higher $T_c$ = liquefies first on cooling.
• Confusing critical temperature with boiling point — they are related but distinct concepts. …