At and above Tc, the sample has a single phase that occupies the entire volume
of the container. Such a phase is, by definition, a gas. Hence, the liquid phase of a
substance does not form above the critical temperature. The critical temperature of
oxygen, for instance, signifies that it is impossible to produce liquid oxygen by compression
alone if its temperature is greater than 155 K: to liquefy oxygen—to obtain a
fluid phase that does not occupy the entire volume—the temperature must first be
lowered to below 155 K, and then the gas compressed isothermally. The single phase
that fills the entire volume when T > Tc may be much denser that we normally consider
typical of gases, and the name supercritical fluid is preferred.