$A$ liquid can exist only,

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 $NH_3: 405.5$)

Why does the boundary between the liquid phase and the gaseous phase disappear upon heating a liquid up to the critical temperature in a closed vessel? In this situation,what will be the state of the substance?

For gases $W, X, Y$ and $Z$,the van der Waals constants are given below. Which gas has the highest critical temperature?

Gas	$a$ (atm $L^2$ mol$^{-2}$)	$b$ ($L$ mol$^{-1}$)
$W$	$4.0$	$0.027$
$X$	$8.0$	$0.030$
$Y$	$6.0$	$0.032$
$Z$	$12.0$	$0.027$

What will be the phase of $CO_2$ at critical pressure $P_C$ and temperatures of $13.1^{\circ}C$,$31.5^{\circ}C$,$50^{\circ}C$,and $30.98^{\circ}C$?

Which among the following gases liquefy easily?