The supercritical state of water: between liquid and gas, a fourth phase?

Phase diagram of water showing the supercritical zone — In the **supercritical zone**, water is neither liquid nor gas: it is a dense, homogeneous fluid, often described as an intermediate state between the two. It appears beyond 374 °C and 22.06 MPa (about 218 atm). Right at the critical point, giant density fluctuations strongly scatter light: this is critical opalescence. In nature, it is not seen directly: it is confined in the depths of the Earth's crust, the oceanic ridges or geothermal boreholes.
Image source: astronoo.com (new window), image generated by AI, public domain.

Scientific summary

The Astronoo article presents supercritical water, a state of matter located beyond the critical point, at 374 °C and 22.06 MPa. In this domain, liquid and gas are no longer distinguished: the fluid acquires a density close to that of a liquid while retaining the diffusivity of a gas. Its unique properties, low viscosity, zero surface tension and tunable dielectric constant, make it a remarkable solvent and reagent. The article describes its natural deposits, oceanic ridges and deep geothermal boreholes, as well as its industrial applications and its possible role in the origin of life.

Is supercritical water really a fourth phase of matter?

This article is devoted to supercritical water, that strange state that water adopts when it simultaneously crosses its critical temperature (374 °C) and critical pressure (22.06 MPa). But is it really a fourth phase of matter, distinct from the solid, liquid and gaseous states? The answer is nuanced. Beyond the critical point, the boundary between liquid and gas disappears: there is no longer any boiling, condensation or meniscus. The fluid forms a single phase, called supercritical, whose density can vary continuously. We cannot therefore speak of a fourth phase in the strict sense, but rather of an intermediate and continuous state. Its properties are nonetheless spectacular: density close to that of a liquid, diffusivity and compressibility close to those of a gas, zero surface tension, very low viscosity. These characteristics explain why supercritical water is omnipresent in the depths of the Earth, where it circulates in the oceanic crust and around magmatic chambers, and why industry uses it to destroy pollutants, extract molecules or produce geothermal energy. The article explores these facets, from mid-ocean ridges to green chemistry processes.

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A little-known but fundamental phase

A well-defined thermodynamic threshold

The supercritical state of water, reached at a temperature of 374 °C and a pressure of 22.06 MPa (≈217.8 atm), is a thermodynamic domain in which water no longer distinguishes between liquid and gas.

A hybrid behavior with many applications

In this zone, it possesses both the density of a liquid and the diffusivity of a gas. This hybrid behavior opens the way to unique physical and chemical properties, making it highly reactive, excellent for dissolving organic and inorganic substances, and widely used in geochemistry, in industrial processes, and even in the exploration of prebiotic conditions.

The physical properties of supercritical water

A fluctuating microscopic structure

Unlike liquid water or steam, supercritical water exhibits significant density fluctuations at small scales, which modify dielectric constants, ionic solvation and thermal transport mechanisms.

A solvent with paradoxical properties

It can dissolve salts like a liquid but behave like a gas for non-polar species. Its thermal conductivity is high, its viscosity low, and its surface tension is zero. These characteristics make it an excellent working fluid or a green solvent in sustainable chemistry processes.

Where can supercritical water be found on Earth?

Extreme conditions naturally brought together

The supercritical state of water can exist only at pressures above 217 atm and temperatures above 374 °C. These conditions are naturally met in certain deep geothermal environments, particularly in the oceanic crust and near magmatic chambers. They are found in particular in the following zones:

Deep hydrothermal systems: mid-ocean ridges (such as the Atlantic) host chimneys known as "black smokers", where the pressure of the water column and the proximity of magma allow water to cross its critical point.
Ultra-deep geothermal boreholes: projects such as the Iceland Deep Drilling Project (IDDP) have reached zones where water is maintained in the supercritical state at more than 4 to 5 km below the Earth's surface.
Active volcanic fields: in volcanic regions (such as Taupo in New Zealand or Yellowstone in the United States), very high geothermal gradients can generate supercritical zones at relatively shallow depths.

A major scientific and technological interest

These extreme environments are of major interest both for fundamental research (origin of life, extremophiles, mineralization) and for emerging geoenergy technologies.

Scientific and industrial applications

A fluid with multiple uses

Supercritical water is found in many fields, where its properties as a solvent and reagent are put to use:

Deep geochemistry: it plays a role in metamorphism, volcanism and the transport of heavy metals.
Waste treatment: in supercritical water oxidation (SCWO), organic pollutants are destroyed efficiently.
Pharmaceutical and agri-food industry: it enables selective extractions without toxic solvents.
Planetary exploration: supercritical oceans are suspected beneath icy crusts or on high-gravity exoplanets.

A key phase in Earth's evolution

A possible role in the origin of life

The supercritical state of water may have played a role in the origin of life on Earth. In deep hydrothermal environments, supercritical water may have favored complex organic reactions, precursors to the appearance of the first biological molecules. Moreover, the exceptional solvation and catalysis properties may have supported the development of prebiotic chemistries on the surface or in the Earth's crust.

Comparative table: states of water

The table below compares the main physical properties of water according to its state: liquid, vapor or supercritical.

Compared properties of water according to its physical state
Property	Liquid	Vapor	Supercritical
Density (kg/m³)	1000	0.6	200-500
Viscosity (μPa·s)	890	13	100-300
Dielectric constant	≈80	≈1.0	≈10-20
Solubility of salts	High	None	Medium to high (variable)
Surface tension	72 mN/m	Zero	Zero

References

FAQ: everything you need to know about supercritical water

What is supercritical water?

Supercritical water is a state of water located beyond its critical point. In this domain, the distinction between the liquid and gaseous phases disappears: only a single fluid remains, whose density can vary continuously. This fluid retains certain properties of the liquid, such as high density and good solvent power, while adopting properties of the gas, such as low viscosity and rapid diffusivity.

What conditions are needed to obtain supercritical water?

It is necessary to simultaneously exceed the critical temperature of 374 °C and the critical pressure of 22.06 MPa, i.e. about 217.8 atmospheres. Below either of these two values, water remains either liquid or gaseous. Beyond that, the phase boundary disappears and water enters the supercritical domain, where it can no longer be liquefied or vaporized by a simple change in pressure or temperature.

Where is supercritical water found in nature?

Supercritical water exists naturally in the depths of the Earth. It is found in the hydrothermal systems of mid-ocean ridges, where the pressure of the water column and the proximity of magma cross the critical point, as well as in ultra-deep geothermal boreholes such as those of the Icelandic IDDP project. Certain active volcanic fields, such as those of Taupo or Yellowstone, also present supercritical zones at relatively shallow depths.

What are the industrial applications of supercritical water?

Supercritical water is used in supercritical water oxidation (SCWO) to destroy organic pollutants, in the pharmaceutical and agri-food industry for selective extractions without toxic solvents, and in deep geochemistry to understand the transport of metals. It is also of interest to the energy sector, where it serves as a working fluid in high-efficiency thermodynamic cycles.

Can supercritical water explain the origin of life?

This is a serious hypothesis. In deep hydrothermal environments, supercritical conditions may have favored complex organic reactions, precursors of the first biological molecules. The exceptional solvation and catalysis properties of this fluid may thus have offered a reaction medium conducive to the emergence of prebiotic chemistries, on the surface as well as in the Earth's crust.

Does supercritical water exist elsewhere in the Solar System?

Several clues suggest that supercritical oceans could exist beneath the icy crusts of certain moons, such as Europa or Enceladus, if the pressure and temperature there reach the required thresholds. On high-gravity exoplanets, the pressure in the deep layers of the oceans could also maintain water in the supercritical state, which opens fascinating perspectives for astrobiology.