Critical Temperature
The temperature above which a gas cannot be liquefied by pressure alone, however great.
Formula / equation
T_c = 8a / 27Rb
Unit
K
In depth
Above T_c the molecules have too much kinetic energy for intermolecular attractions ever to hold them in a liquid, so the distinction between liquid and gas disappears. Critical temperature rises with the strength of intermolecular forces, which is why polar substances have high values.
Examples in the real world
CO₂ has T_c = 31 °C and liquefies in a cylinder on a cool day; nitrogen must be cooled below −147 °C first.