Water Phase Diagram

Phase diagrams
The phase diagram of water
Density change
Triple points
The ice phases

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) on a phase diagram represents a phase boundary and gives the conditions when two phases may stably coexist (in any relative proportions). 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 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, liqiuid 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 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 has been described [1320].

Water phase diagram

The mean surface conditions on Earth, Mars and Venus 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 and the boundary between supercritical water and ice-seven (VII) (see [691 ]) is still to be firmly established. Ice VII possesses higher and lower pressure forms [1428 ].

Both the critical points are shown as red circles in the phase diagram, above. 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 ], where physical properties, such as gas-like or liquid-like behavior, vary in response to changing density. 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 ]. The properties of supercritical water^c 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-ionization 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. 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 ionization, solubility, diffusivity and reactivity due to decreasing hydrogen-bonding [1116 ]. [Back to Top ]

3-D Pressure-Temperature-Density graph, liquid-gas data derived from ref 540

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.

2-D Pressure-Temperature-Density graph, liquid-gas data derived from ref 540

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].

Variation in the density of liquid, gaseous and supercritical water with emperature and pressure

Density of liquid and solid water along the phase line

The density of liquid water increases with increase in pressure.

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). 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. [Back to Top ]

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	ΔS, J mol^-1 K^-1	ΔV cm³ mol^-1	Ref.	D₂O [711 ]
gas	liquid	Ih	0.000611657	0.010			536	661 Pa, 3.82°C [70]
			gasliquid		-132.5	-22050
			gasIh		-154.5	-22048
			liquidIh		-22.0	1.634	8
gas	Ih	XI	0	-201.0			711	0 MPa, -197°C
liquid	Ih	III	209.9	-21.985			537	220 MPa, -18.8°C
			liquidIh		-14.9	2.434	838
			liquidIII		-13.9	-0.839
			IhIII		1.0	-3.273
Ih	II	III	212.9	-34.7			537	225 MPa, -31.0°C
			IhII		-2.1	-3.919	838
			IhIII		1.0	-3.532
			IIIII		3.2	0.387
II	III	V	344.3	-24.3			537	347 MPa, -21.5°C
			IIIII		3.1	0.261	838
			IIV		3.3	-0.721
			IIIV		0.1	-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		-13.2	-0.434	838
			liquidV		-13.1	-1.419
			IIIV		0.1	-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		-15.7	-0.949	838
			liquidVI		-16.2	-1.649
			VVI		-0.5	-0.700
VI	VIII	XV					1582	~1.5 GPa, -143°C
VI	VII	VIII	2,100	~5			8	1950 MPa, ~0°C
liquid	VI	VII	2,216	81.85			537	2060 MPa, 78°C
VII	VIII	X	62,000	-173			538
liquid	VII	X	43,000	>700			612a
liquid	VII	X	47,000	~727			612b
liquid	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. [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]

^cSupercritical water presents a reactive environment [1507 ]. 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, sheetlike configurations [1508 ]. Under extreme conditions (e.g. 2.38 g cm^-3, 3000 K), dense hot water may be extremely reactive [1564 ]. [Back]