Which intermolecular force is most responsible for allowing $Xe$ gas to liquefy?

Explain the liquefaction of $CO_2$ gas using the Thomas Andrews plot,including the effects of temperature and pressure,and define the critical constants.

What are the most favourable conditions to liquefy a gas?

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.

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 $NH_3: 405.5$)

Explain the liquefaction of real gases and permanent gases.