$1.$ What is critical phenomena in the context of phase equilibria, and why is it important in thermodynamics?

$2.$ How can critical temperature and pressure be determined experimentally for a pure substance?

$3.$ Define fugacity and its significance in thermodynamic calculations.

$4.$ How does fugacity differ from pressure, and why is it a useful concept in non$-$ideal systems?

$5.$ Explain how fugacity is used to describe the departure from ideal gas behavior.