The cooling in a refrigerator is due to

If $Z$ is a compressibility factor,the van der Waals equation at low pressure can be written as:

The compressibility factor $(Z)$ of a gas is less than unity at $STP$,then:

The compressibility factor $Z = \frac{pV}{nRT}$ for hydrogen gas at $273 \ K$ and $1 \ atm$ pressure is

For real gases,the relation between $p$,$V$,and $T$ is given by the van der Waals equation:
$(p + \frac{an^2}{V^2})(V - nb) = nRT$
Where $a$ and $b$ are van der Waals constants,$nb$ is approximately equal to the total volume of the molecules of a gas,and $a$ is the measure of the magnitude of intermolecular attraction.
$(i)$ Arrange the following gases in the increasing order of $b$. Give reason: $O_2, CO_2, H_2, He$
$(ii)$ Arrange the following gases in the decreasing order of magnitude of $a$. Give reason: $CH_4, O_2, H_2$

In van der Waals equation of state,the constant $b$ is a measure of