Water Phase Diagram

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

Phase diagrams
The phase diagram of water

'Corresponding to the abnormal behavior of the liquid at low pressures, and probably connected to it, the solid also shows abnormal behavior...'

Percy Bridgman, 1911 [2472]

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. At temperatures above the critical temperature a gas cannot be liquefied. 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^{, c}^{, d, 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). It is noticeable that most phase boundaries between the ices that share phase boundaries, particularly with the liquid, are parallel to the temperature axis, implying density-driven phase transformations [2465]; entropy-driven phase transformations showing phase boundaries parallel to the pressure axis. 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] .

Water phase diagram

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 ].]

Triple points

Triple points occur where three phase lines join and the three (stable) 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
gas	Ih	XI	Ih XI				0
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	XI	70	-199.8				2300	80 MPa, -195 °C [2300]
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
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
super-critical fluid	VII	X	43,000	>700				612a
super-critical fluid	VII	X	47,000	~727				612b
super-critical fluid	VII	Superionic	~40,000	~1000				1572

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Footnotes

^aGaseous 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]

Phase diagram of water, if it behaved like a more typical material of its molecular weight

^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]

^cThe metastable phases (Ice Ic, ice IV, Ice IX and Ice XIII) do not belong in a phase diagram. However, Ice Ic (cubic ice) has been found in the hexagonal ice phase space below -80 °C, ice IV has been found within the ice-three, ice-five and ice-six phase spaces and Ice IX and Ice XIII have been found within the ice-two phase space. As with the stable ice phases, at low enough temperatures these phases may be stable for extensive periods as there is insufficient energy available to overcome the necessary activation barriers. [Back]

^d Note that phase diagrams originating from this site have been copied by various sources including to and from Wikipedia without proper citation of their source. Such poor scientific etiquette is easy for me to detect and reflects badly on these authors. [ 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]

^fThe 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]