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Black smoker chimney showing the exit of supercritical water together with dissolved minerals. National Oceanic and Atmospheric Administration; edited


Water is supercritical above its critical point (>647.096 K, >22.064 MPa) where liquid and gas phases cannot be distinguished.

Supercritical water

      'High temperature aqueous physical chemistry is fertile ground for

              scientific and engineering advances'

  Ernst Ulrich Franck, 2004


Beyond the critical point (>647.096 K, >22.064 MPa) in the liquid-vapor space (towards the top right of the water phase diagram below), water is supercritical existing as small but liquid-like hydrogen-bonded clusters dispersed within a gas-like phase [456, 894, 1962], where physical properties, such as gas-like or liquid-like behavior, vary in response to changing temperature, pressure and density and the normal distinction between gas and liquid has disappeared [1766]. At the critical point there is about 30% free monomeric H2O molecules and only about 17% hydrogen bonding; but this is far greater than in the gas phase. It is heterogeneous, and instantaneous properties vary over a wide range between the different structural forms, giving 'average' properties that are, perhaps, not representative. As with other supercritical fluids. supercritical water has no surface tension with its gas or liquid phase or any other supercritical phase as no such interfaces exist. Above 647.096 K, water vapor cannot be liquefied by increasing pressure.


Water phase diagram showing the supercritical stateThe properties of supercritical water are very different from ambient liquid water. For example, supercritical water is a poor solvent for electrolytes, which tend to form ion-pairs. However, it is such an excellent solvent for non-polar molecules, due to its low relative permittivity (dielectric constant) and poor hydrogen bonding, that many are completely miscible. Viscosity and dielectric both decrease substantially whereas auto-dissociation increases substantially. The physical properties of water close to the critical point (near-critical) are particularly strongly affected [677] by density. Extreme density fluctuations around the critical point causes opalescent turbidity. Neutron diffraction has shown that tetrahedral liquid-like states are seen in supercritical water at above a threshold density, while below this threshold density gas-like water forms small, trigonal, sheet-like configurations [1508]. From
the dependence of the frequency maximum of the Raman O-H stretching bands, at about 3500 cm-1, with increasing temperature and pressure, the structure of supercritical water can be divided between three-dimensional and two-dimensional clustering corresponding to liquid- and gas-like states [2896]. No fully-tetrahedral states were found in supercritical water.


Supercritical water has total miscibility with organic fluids and oxygen in supercritical equilibria and presents a reactive environment [1507] capable of oxidizing toxic waste. Under extreme conditions (e.g. 2.38 g cm-3, 3000 K), it may be extremely reactive [1564] causing explosive reactions. The wide range of inorganic material solubility under varying temperature and pressure gives rise to many synthetic processes. As with other supercritical fluids, supercritical water has high compressibility and low viscosity.


The critical isochor (density 322 kg m-3; shown as the thin dotted line extension in the phase diagram above) may be thought of as dividing more-liquid-like and more-gas-like properties [540]. This isochor is outwardly similar to the loci of the CP maximum, and the thermal expansion and compressibility maxima, (the 'Widom' line [1715]). Above the line is a more 'liquid-like' material and below the line is a more 'gas-like' environment. However, the Widom lines for isobaric heat capacity, isochoric heat capacity, isothermal compressibility, isobaric thermal expansion, mass density and the molar internal energy all differ, particularly above 800 K, 100 MPa [1923]. A case has been put for a different line (the `Frenkel line') that separates liquid and gas-like fluids on their dynamic properties above the critical point [1961 ]; for example, viscosity, the speed of sound, and thermal conductivity all decrease with increasing temperature above the Frenkel line, as in liquids, but increase with temperature when sufficiently below the line, as in gases [1961]. Use of a multi-parameter electronically coarse-grained model for water has shown both gas-like (dissociated) and liquid-like (associated) water molecules with the density dependence of the molecular dipole moment distinguishing the hydrogen-bond connectivity [2398 ].

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This page was established in 2015 and last updated by Martin Chaplin on 14 May, 2017

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