Anomalies of water

Water: The thirty anomalies of water¹

Water has unusually high melting point. [explanation]
Water has unusually high boiling point. [explanation]
Water has unusually high critical point. [explanation]
Water has unusually high surface tension. [explanation]
Water has unusually high viscosity. [explanation]
Water has unusually high heat of vaporization. [explanation]
Water shrinks on melting. [explanation]
Water has a high density that increases on heating (up to 3.984ƒC). [explanation]
The number of nearest neighbors increases on melting. [explanation]
The number of nearest neighbors increases with temperature. [explanation]
Pressure reduces its melting point (13.35 MPa gives a melting point of -1ƒC) [explanation]
Pressure reduces the temperature of maximum density. [explanation]
D₂O and T₂O differ from H₂O in their physical properties much more than might be expected from their increased mass; e.g. they have increasing temperatures of maximum density (11.185ƒC and 13.4ƒC respectively). [explanation]
Water shows an unusually large viscosity increase as the temperature is lowered. [explanation]
Water's viscosity decreases with pressure (at temperatures below 33ƒC). [explanation]
Water has unusually low compressibility. [explanation]
The compressibility drops as temperature increases (below a minimum at about 46.5ƒC). [explanation]
Water has a low coefficient of expansion (thermal expansivity). [explanation]
Water's thermal expansivity reduces increasingly (becoming negative) at low temperatures. [explanation]
The speed of sound increases with temperature (up to a maximum at 73ƒC). [explanation]
Water has over twice the specific heat capacity of ice or steam. [explanation]
The specific heat capacity (C_P and C_V) is unusually high. [explanation]
The specific heat capacity (C_P) has a minimum (36ƒC). [explanation]
NMR spin-lattice relaxation are very small at low temperatures. [explanation]
Solutes have varying effects on properties such as density and viscosity. [explanation]
None of its solutions even approach thermodynamic ideality; even D₂O in H₂O is not ideal. [explanation]
X-ray diffraction shows an unusually detailed structure. [explanation]
Supercooled water has two phases and a second critical point at -45ƒC. [explanation]
Liquid water is easily supercooled down to about -25ƒC and with more difficulty down to about -38ƒC with further supercooling possible, in tiny droplets, down to about -70ƒC. Supercooled liquid water can be produced from glassy amorphous ice between -123ƒC and - 149ƒC [74] and may coexist with cubic ice up to -63ƒC [137]. [explanation]
Solid water exists in a wider variety of stable (and metastable) crystal and amorphous structures than other materials. [explanation]

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Some of the anomalies of liquid water related to temperature. The graph uses data that have been scaled between their maximum and minimum values (original data).

The anomalies of water are useful for life

It has often been stated (e.g. [127]) that life depends on the anomalous properties of water. In particular, the large heat capacity and high water content in organisms contribute to thermal regulation and prevent local temperature fluctuations. The high latent heat of evaporation gives resistance to dehydration and considerable evaporative cooling. Water is an excellent solvent due to its polarity, high dielectric constant and small size, particularly for polar and ionic compounds and salts.² It has unique hydration properties towards biological macromolecules (particularly proteins and nucleic acids) that determine their three-dimensional structures, and hence their functions, in solution. Water ionizes and allows easy proton exchange between molecules, so contributing to the richness of the ionic interactions in biology.

The density maximum at 4ƒC and low ice density results in (i) the necessity that all of a body of water (not just its surface) is close to 0ƒC before any freezing can occur, (ii) the freezing of rivers, lakes and oceans is from the top down, so insulating the water from further freezing and allowing rapid thawing, and (iii) density driven thermal convection causing seasonal mixing in deeper temperate waters. The large heat capacity of the oceans and seas allows them to act as heat reservoirs such that sea temperatures vary only a third as much as land temperatures and so moderate our climate. The compressibility of water reduces the sea level by about 40 m giving us 5% more land. [65]

¹ Whether or not the properties of water are seen to be anomalous depends upon which materials water is to be compared and the interpretation of 'anomalous'. For example, it could well be argued that water possesses exactly those properties that one might deduce from its structure. Comparisons between water, liquid sodium, argon and benzene appeared to Franks [112] to indicate several of the properties given above as not being anomalous. However these materials are perhaps not the most typical of liquids. The list gives the properties generally understood to make liquid water (and in one case ice) stand out from 'typical' liquids (or in one case solids). [Back]

² It is therefore difficult to obtain really pure water. [Back]

Please submit comments and any other suggestions of water anomalies

This page was last updated by Martin Chaplin
on 08 September 2000