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The anomalous properties of water are those where the behavior of liquid water is quite different from what is found with other liquids [1414]. a No other material is commonly found as solid, liquid and gas. dFrozen water (ice) also shows anomalies when compared with other solids. Although it is an apparently simple molecule (H2O), it has a highly complex and anomalous character due to its inter-molecular hydrogen bonding (see [1530] for example). As a gas, water is one of lightest known, as a liquid, it is much denser than expected and as a solid, it is much lighter than expected when compared with its liquid form. It can be extremely slippery and extremely sticky at the same timef, and this 'stick/slip' behavior is how we recognize the feel of water [2411]. Many other anomalies of water may remain to be discovered, such as the possible link of water to room temperature superconductivity [2124]. h An interesting history of the study of the anomalies of water has been published [1542].
As liquid water is so common-place in our everyday lives, it is often regarded as a ‘typical’ liquid. In reality, water is most atypical as a liquid, behaving as a quite different material at low temperatures to that when it is hot, with a division temperature of about 50 °C. It has often been stated (for example, [127]) that life depends on these anomalous properties of water. The anomalous macroscopic properties of water are derived from its microscopic structuring.
The high cohesion between molecules gives it a high freezing and melting point, such that we and our planet are bathed in liquid water. The large heat capacity, high thermal conductivity and high water content in organisms contribute to thermal regulation and prevent local temperature fluctuations, thus allowing us to more easily control our body temperature. The high latent heat of evaporation gives resistance to dehydration and considerable evaporative cooling. It has unique hydration properties towards important biological macromolecules (particularly proteins and nucleic acids) that determine their three-dimensional structures, and hence their biological functions, in solution. This hydration forms gels that can reversibly undergo the gel-sol phase transitions that underlie many cellular mechanisms [351]. Water ionizes and allows easy proton exchange between molecules, so contributing to the richness of the ionic interactions in biology. Also, it is an excellent solvent due to its polarity, high relative permittivity (dielectric constant) and small size, particularly for polar and ionic compounds and salts. b
At 4 °C water expands on heating or cooling. This density maximum together with the low ice density results in (i) the necessity that all of a body of fresh water (not just its surface) is close to 4 °C before any freezing can occur, (ii) the freezing of rivers, lakes, and oceans is from the top down, so permitting survival of the bottom ecology, insulating the water from further freezing, reflecting back sunlight into space and allowing rapid thawing, and (iii) density driven thermal convection causing seasonal mixing in deeper temperate waters carrying life-providing oxygen into the depths. 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 planet's climate (for example, the Gulf stream carries tropical warmth to northwestern Europe). The compressibility of water reduces the sea level by about 40 m giving us 5% more land [65]. Water's high surface tension plus its expansion on freezing encourages the erosion of rocks to give soil for our agriculture.
Notable amongst the anomalies of water is the opposite properties of hot and cold water, with the anomalous behavior more accentuated at low temperatures where the properties of supercooled water often diverge from those of hexagonal ice. In particular, several properties of water change at about 50 °C [2755]; just above the body temperature of mammals and about which many proteins denature. As (supercooled) cold liquid water is heated individual molecules shrink, bulk water shrinks and becomes less easy to compress, its refractive index increases, the speed of sound within it increases, gases become less soluble and it is easier to heat and conducts heat better. In contrast, as hot liquid water is heated it expands, it becomes easier to compress, its refractive index reduces, the speed of sound within it decreases, gases become more soluble and it is harder to heat and a poorer conductor of heat. With increasing pressure, individual molecules expand, cold water molecules move faster but hot water molecules move slower. Hot water freezes faster than cold water and ice melts when compressed except at high pressures when liquid water freezes when compressed.
Fluctuations in liquid water i