The liquid, vapor critical point of water occurs at 374 degrees Celsius and 218 atmospheres. At this precise temperature and pressure, the boundary between liquid and vapor vanishes completely. Phase boundaries disappear when a substance reaches its critical state. One example is the end point of a phase equilibrium curve. At higher temperatures, the gas comes into a supercritical phase. It cannot be liquefied by pressure alone. The heat of vaporization becomes zero at this exact moment. There is a stationary inflection point in the constant-temperature line on a PV diagram. This means that at the critical point, only one phase exists.
Historical Discovery And Naming
Charles Cagniard de la Tour first discovered the existence of a critical point in 1822. He showed that carbon dioxide could be liquefied at 31 degrees Celsius at a pressure of 73 atmospheres. He found it would not liquefy even under pressures as high as 3000 atmospheres if the temperature was slightly higher. Dmitri Mendeleev named this phenomenon the absolute temperature of boiling in 1860. In 1870, Mendeleev asserted his priority regarding the definition against Thomas Andrews who published work in 1869. The term critical point appears on page 588 of later texts. These experiments established the fundamental understanding of how substances behave near their limits.Anomalous Fluid Properties
Liquid water under normal conditions is nearly incompressible and has a low thermal expansion coefficient. Near the critical point, all these properties change into the exact opposite. Water becomes compressible and expandable while acting as a poor dielectric. It transforms from an excellent solvent for electrolytes to a bad solvent for them. The substance mixes more readily with nonpolar gases and organic molecules. Anisimov, Sengers, and Levelt Sengers documented this behavior in 2004 within Aqueous System at Elevated Temperatures and Pressures. Sometimes the critical point does not manifest in most thermodynamic or mechanical properties but reveals itself in the onset of inhomogeneities in elastic moduli. Marked changes appear in the appearance and local properties of non-affine droplets. A sudden enhancement occurs in defect pair concentration during these transitions.