$1.$ The concept of $\_\_\_\_\_\_\_\_\_$ is used to quantify the deviation of real gas behavior from ideal gas behavior.

$2.$ Critical phenomena occur at the $\_\_\_\_\_\_\_\_\_$ point, where distinct liquid and gas phases become indistinguishable.

$3.$ The $\_\_\_\_\_\_\_\_\_$ rule is applied to determine the number of degrees of freedom in a multiphase, multicomponent system.

$4.$ Fugacity becomes equivalent to pressure in the limit of an $\_\_\_\_\_\_\_\_\_$ gas.