Critical point
The end of the liquid–vapour boundary, above which liquid and gas become a single indistinguishable supercritical fluid.
Definition
The critical point is the temperature and pressure at which the liquid–vapour coexistence curve terminates. Beyond it — above both the critical temperature and critical pressure — there is no distinction between liquid and gas: the substance becomes a single supercritical fluid that can be compressed continuously from gas-like to liquid-like densities without ever crossing a phase boundary. For water the critical point is 374 °C and 22.06 MPa.
Approaching the critical point, the difference in density between liquid and vapour shrinks to zero and fluctuations grow without bound, producing striking effects such as critical opalescence, where the fluid turns milky as density fluctuations scatter light. These divergences are the domain of critical phenomena and universality.
Supercritical fluids have practical uses — supercritical CO₂ is a tunable, residue-free solvent for decaffeinating coffee and extracting essences. This entry is a stub; the critical point and critical phenomena are explored in depth later in the branch.
History
Discovered by Charles Cagniard de la Tour (1822) and named and characterised by Thomas Andrews (1869) in his studies of carbon dioxide; later central to the theory of critical phenomena.