Liquefaction of a gas can occur $....................$

Gases possess characteristic critical temperature which depends upon the magnitude of intermolecular forces between the gas particles. Critical temperatures of ammonia and carbon dioxide are $405.5 \ K$ and $304.10 \ K$ respectively. Which of these gases will liquefy first when you start cooling from $500 \ K$ to their critical temperature?

What is meant by the vapor of a substance like $CO_2$?

What are the critical constants?

Which are the most favorable conditions for the liquefaction of a gas?

At which of the following conditions can a gas be liquefied? ($T_c$ and $P_c$ are critical temperature and pressure.)