When $He$ gas is expanded into a vacuum,a heating effect is observed. What is the reason for this?

The units of constant $a$ in the Van der Waals equation are:

The Van der Waals parameters for gases $W$,$X$,$Y$,and $Z$ are given below. Which one of these gases has the highest critical temperature?

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

Gases deviate from ideal gas behavior because the molecules $...$

At low pressures,van der Waals' equation is written as $(P + \frac{a}{V^2})V = RT$. The compressibility factor $Z$ will be

$1 \ mol$ of a real gas is kept at a high pressure of $100 \ bar$ at $300 \ K$. If the van der Waals constant $b$ is $0.005 \ L \ mol^{-1}$, what are the values of the compressibility factor $Z$ of the gas and the $\%$ deviation of volume from ideality?

$Z$	$\%$ Deviation