Explain the liquefaction of gases with the help of an isothermal graph.

(N/A) The first complete data on the pressure-volume-temperature relations of a substance in both gaseous and liquid states was obtained by Thomas Andrews for $CO_2$.
From the analysis of the Andrews isotherms:
$(i)$ At high temperatures,isotherms resemble those of an ideal gas,and the gas cannot be liquefied even at very high pressures.
$(ii)$ As the temperature is lowered,the shape of the curve changes,and the data shows a significant deviation from ideal behavior.
$(iii)$ Explanation of $T_C$ (Critical Temperature),$P_C$ (Critical Pressure),and $V_C$ (Critical Volume): At $30.98^{\circ}C$,$CO_2$ remains a gas up to $73 \ atm$ pressure (Point $E$ in the figure). At $73 \ atm$ pressure,liquid $CO_2$ appears for the first time. This point $E$ represents the critical state where the distinction between liquid and gas disappears.