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 $NH_3: 405.5$)

Assertion : Gases do not liquefy above their critical temperature,even on applying high pressure.
Reason : Above critical temperature,the molecular speed is high and intermolecular attractions cannot hold the molecules together because they escape because of high speed.

Critical temperatures for $A$,$B$,$C$ and $D$ gases are $25\,^{\circ}C$,$10\,^{\circ}C$,$-80\,^{\circ}C$ and $15\,^{\circ}C$ respectively. Which gas will be liquefied more easily?

$A$ gas can be liquefied:

What is the Andrews isotherm graph used for?

One of the assumptions of the kinetic theory of gases states that "there is no force of attraction between the molecules of a gas." How far is this statement correct? Is it possible to liquefy an ideal gas? Explain.