Liquefaction of Gases

Critical temperature of ${\mathrm{H}}_2\mathrm{O}, {\mathrm{NH}}_3$, and ${\mathrm{CO}}_2$ are $647.4, 405.5$ and $304.2 \mathrm{K}$, respectively. When we start cooling from a temperature of $700 \mathrm{K}$ which will liquefy first and which will liquefy finally?

Can a Van der Waal's gas with $\mathrm{a}=0$ be liquefied? Explain.

Four gases $\mathrm{P}, \mathrm{Q}, \mathrm{R}$ and $\mathrm{S}$ have almost same values of '$\mathrm{b}$' but their '$\mathrm{a}$' values ($\mathrm{a}, \mathrm{b}$ are Van der Waals Constants) are in the order $\mathrm{Q} < \mathrm{R} < \mathrm{S} < \mathrm{P}$. At a particular temperature, among the four gases the most easily liquefiable one is

The table indicates the value of van der Waals constant ‘$\mathrm{a}$' in ${\left({\mathrm{dm}}^3\right)}^2\mathrm{atm}.{\mathrm{mol}}^{-2}$

The gas which can be most easily liquefied is

When an ideal gas undergoes unrestrained expansion, no cooling occurs because the molecules