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# Water Phase Diagram

The properties of the all the known different phases of water are described.

## Phase diagrams

Phase diagrams show the preferred physical states of matter at different temperatures and pressure. Within each phase, the material is uniform with respect to its chemical composition and physical state. At typical temperatures and pressures on Earth (marked by an 'E' below) water is a liquid, but it becomes solid (that is, ice) if its temperature is lowered below 273 K and gaseous (that is, water vapor)a if its temperature is raised above 373 K, at the same pressure. Each line (phase line)e on a phase diagram represents a phase boundary and gives the conditions when two phases may stably coexist in any relative proportions (having the same Gibbs free energy). Here, a slight change in temperature or pressure may cause the phases to abruptly change from one physical state to the other. Where three phase lines join, there is a 'triple point', when three phases stably coexist (having identical Gibbs free energies),e but may abruptly and totally change into each other given a slight change in temperature or pressure. Under the singular conditions of temperature and pressure where liquid water, gaseous water and hexagonal ice stably coexist, there is a 'triple point' where both the boiling point of water and melting point of ice are equal. Four phase lines cannot meet at a single point. A 'critical point' occurs at the end of a phase line where the properties of the two phases become indistinguishable from each other, for example when, under singular conditions of temperature and pressure, liquid water is hot enough and gaseous water is under sufficient pressure that their densities are identical. Critical points are usually found at the high temperature end of the liquid-gas phase line.

## The phase diagram of water

The phase diagram of water is complex,b ,f having a number of triple points and one, or possibly two, critical points. Many of the crystalline forms may remain metastable in much of the low-temperature phase space at lower pressures. A thermodynamic model of water and ices Ih, III, V and VI [1320] and thermodynamic functions of the phase transitions [1658] have been described. The known ices can be divided, by cluster analysis of their structures [1717], into the low-pressure ices (hexagonal ice, cubic ice and ice-eleven). the high pressure ices (ice-seven, ice-eight and ice-ten) and the others (found in the relatively narrow range of moderate pressures between about 200-2000 MPa). All phases that share phase boundaries with liquid water (ices Ih, III, V and VI and VII) have disordered hydrogen bonding. The phases with ordered hydrogen bonding are found at lower temperatures and are indicated in light blue below. The structural transformation conditions of some of these ices during compression have been described [1795] .

Mouse over border for D2O phase lines

The mean surface conditions on Earth, (also atmospheric conditions, Mars and Venus on mousing over Earth) are indicated. The complex central part of the phase diagram is expanded opposite. The critical point and the orange line in the ice-one phase space refer to the low-density (LDA) and high-density (HDA) forms of amorphous water (ice) [16]. Although generally accepted and supported by diverse experimental evidence [754a, 861], the existence of this second, if metastable, critical point is impossible to prove absolutely at the present time and is disputed by some [200, 618, 628, 754b, 1115]. The transition between LDA and HDA is due to the increased entropy and attractive van der Waals contacts in HDA compensating for the reduced strength of its hydrogen bonding.

The high-pressure phase line between ice-ten (X) and ice-eleven (XI) [81] is still subject to experimental verification. The melting point line between supercritical water and high pressure ice has been established [691, 2096]. Ice VII possesses higher and lower pressure forms [1428]. A phase diagram of water at higher temperatures, up to 9000 K, has been proposed [1671].

Both the critical points are shown as red circles in the phase diagram, above.

Many properties of cold liquid water change above about 200 MPa (for example, viscosity, self-diffusion, compressibility, Raman spectra and molecular separation), which may be explained by the presence of a high density liquid phase containing interpenetrating hydrogen bonds.  The chemical properties of water are also greatly changed at high temperatures and pressures due to the changes in dissociation, solubility, diffusivity and reactivity due to decreasing hydrogen-bonding [1116].

## Supercritical water

Beyond the critical point in the liquid-vapor space (towards the top right, above), 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 density and the normal distinction between gas and liquid has disappeared [1766] . The critical isochor (density 322 kg m-3) is shown as the thin dashed line extension; this may be thought of as dividing more-liquid-like and more-gas-like properties [540].d The properties of supercritical water are very different from ambient 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 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], Extreme density fluctuations around the critical point causes opalescent turbidity. Supercritical water presents a reactive environment [1507] and under extreme conditions (e.g. 2.38 g cm-3, 3000 K), dense hot water may be extremely reactive [1564]. Neutron diffraction has shown that tetrahedral liquid-like states are observed in supercritical water at above a threshold density, while below this threshold density gas-like water forms small, trigonal, sheet-like configurations [1508].

## Density changes

As pressure increases, the ice phases become denser. They achieve this by initially bending bonds, forming tighter ring or helical networks, and finally including greater amounts of network inter-penetration. This is particularly evident when comparing ice-five with the metastable ices (ice-four and ice-twelve) that may exist in its phase space.

The liquid-vapor density data for the graphs above, opposite and below were obtained from the IAPWS-95 equations [540].

Other phase diagrams for water are presented elsewhere [681]. The density of supercooled (emulsified) water under pressure has recently been determined [1685 ].

The two graphs below show the variation in the density of liquid, gaseous and supercritical water with temperature and pressure. The density of liquid water increases with increasing pressure and decreases with increasing temperature.

Seen opposite is the density of liquid and solid (that is the ices) water along the liquid-solid phase line. Note that temperature varies along this phase line (as shown dashed red). Hexagonal ice is less dense than liquid water whereas the other ices found in equilibrium with water are all denser with phase changes occurring on the approach of the liquid and solid densities.

## Triple points

Triple points occur where three phase lines join and the three phases may coexist at equilibrium.

Thermodynamic data for the triple points of water
Triple points MPa °C ΔH, kJ mol-1

ΔS,

J mol-1 K-1

ΔV cm3 mol-1 Ref. D2O [717]
gas liquid Ih 0.000611657 0.010   536 661 Pa, 3.82 °C [70]
gasliquid
-44.9 -165 -22050 1833
gasIh
-50.9 -186 -22048
liquidIh
-5.98 -22 1.634
gas Ih XI 0 -201.0   717 0 MPa, -197°C
liquid Ih III 209.9 -21.985   537 220 MPa, -18.8°C
liquidIh
-4.23 -16.9 2.434 1833
liquidIII
-3.83 -15.3 -0.839
IhIII
0.39 1.6 -3.273
Ih II III 212.9 -34.7   537 225 MPa, -31.0°C
IhII
-0.75 -3.2 -3.919 1833
IhIII
0.17 0.7 -3.532
IIIII
0.92 3.8 0.387
II III V 344.3 -24.3       537 347 MPa, -21.5°C
IIIII
1.27 5.1 0.261 1833
IIV
1.20 4.8 -0.721
IIIV
-0.07 -0.2 -0.982
II VI XV       1582 ~0.8 GPa, -143°C
liquid III V 350.1 -16.986   537 348 MPa, -14.5°C
liquidIII
-4.61 -18.0 -0.434 1833
liquidV
-4.69 -18.3 -1.419
IIIV
-0.07 -0.2 -0.985
liquid IV XII ~500-600 ~-6   1300
II V VI ~620 ~-55   539
liquid V VI 632.4 0.16   537 629 MPa, 2.4°C
liquidV
-5.27 -19.3 -0.949 1833
liquidVI
-5.29 -19.4 -1.649
VVI
-0.02 -0.5 -0.700
VI VIII XV       1582 ~1.5 GPa, -143°C
VI VII VIII 2,100 ~5   8 1950 MPa, ~0°C
VIVII -0.09 -o.3 -1.0 1833
VIVIII -1.20 -4.2 -1.0
VIIVIII -1.10 -3.9 0.0
liquid VI VII 2,216 81.85   537 2060 MPa, 78°C
liquidVI -6.36 -18.0 -0.59 1833
liquidVII -6.36 -18.0 -1.64
VIVII 0.0 0.0 -1.05
VII VIII X 62,000 -173   538
liquid VII X 43,000 >700   612a
47,000 ~727   612b
liquid VII Superionic ~40,000 ~1000   1572

## Footnotes

a Gaseous water is water vapor. In science and engineering, the word 'steam' is also used for water vapor, but usually when above the boiling point of water. As commonly used in the English language, 'steam' also may mean the white cloud of fine liquid water droplets of condensed water vapor that is produced by a boiling kettle, for example. Water is present in the atmosphere in both liquid and gaseous forms. For example, when we breathe out we expire an aerosol of fine (nm - µm+ radius) water droplets plus water vapor. This aerosol has been detected by a water-cluster-detecting breath sensor developed for detecting drunk or drowsy drivers [1801]. [Back]

b If water behaved more typically as a low molecular weight material, its phase diagram may have looked rather like this (where 'x' marks ambient conditions on earth). [Back]

d 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 at higher pressures [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]. [Back]

e On a phase line the Gibbs free energies of the two phases (G1, G2) must be equal and remain equal if conditions change causing movement along the phase line. Thus

But, where S is the entropy, and where V is the volume

Therefore where L is the latent heat (enthalpy change) for the phase change. This is the Clausius-Clapeyron equation. At a triple point, the Gibbs free energies of the three phases (G1, G2, G3) must be equal and the entropy and enthalpy (latent heat) changes for all three phase changes (12, 23, 13) at that point may be calculated, given the pressure, temperature () and volume changes. [Back]

f The phase diagram for heavy water (D2O) differs little from the diagram for H2O given the scales used in the diagram. A more accurate representation would be by shifting the temperature scale by about 3.6 K as most of the triple points for D2O are 3 - 4 K warmer than those for H2O (see above). The pressure differences are positive or negative but not significant given the logarithmic scale used. [Back]

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This page was last updated by Martin Chaplin on 6 August, 2014