Four gas cylinders containing $He$,$N_2$,$CO_2$ and $NH_3$ gases separately are gradually cooled from a temperature of $500 \ K$. Which gas will liquefy first?
(Given $T_C$ in $K - He: 5.3, N_2: 126, CO_2: 304.1$ and $NH_3: 405.5$)

At what pressure does $SO_2$ gas liquefy at room temperature?

What happens when the temperature of $CO_2$ at its critical temperature of $30.98 \, ^\circ C$ is increased or decreased?

Gases possess characteristic critical temperature which depends upon the magnitude of intermolecular forces between the particles. Following are the critical temperatures of some gases.

$Gases$	$H_2$	$He$	$O_2$	$N_2$
$Critical \ temperature \ (K)$	$33.2$	$5.3$	$154.3$	$126$

From the above data,what would be the decreasing order of liquefaction of these gases?

Which among the following gases is difficult to liquefy?

The compressibility factor $(Z)$ of a gas at critical state is $(T_c = \frac{8 a}{27 R b}; p_c = \frac{a}{27 b^2}; V_c = 3 b)$.