As recent models predicted, they saw molecules 0.28 and 0.45 nanometers apart. These measurements confirm the current commonly accepted model, which describes liquid water as a group of water molecules held together in tetrahedral shapes, with the molecule at the center of the tetrahedron separated from four others at the shorter distance and each of these four molecules separated from one another at the longer distance. Yet the researchers saw some molecules at a third distance as well: 0.34 nanometers. The existence of this third separation length, though not included in the current model, was first seen in 1938. Additional experiments in the 1960s and 1970s first confirmed, but later rejected, that this length exists, concluding that its detection was due to shortcomings in the analysis. As a result, models including this intermediate distance fell out of favor—until now.
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Fully understanding the structure of water is a surprisingly difficult task. In water's solid form, ice, the molecules are known to form a tight tetrahedral lattice. The current model holds that liquid water should be similar to ice but less structured since heat creates disorder and breaks bonds. In liquid water, then, the tetrahedral structures would loosen their grip, breaking apart as the temperature rises, but still inclined to remain as tetrahedral as possible. This new research adds a kink in this theory, requiring some sort of secondary structure. The greater density of liquid water implies that the molecules are more closely packed than the simple tetrahedra seen in ice. These data help explain how that can happen.

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.

It has often been stated (for example, [127]) that life depends on these anomalous properties of water. In particular, 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.

Water is an excellent solvent due to its polarity, high dielectric constant and small size, particularly for polar and ionic compounds and salts.b 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. 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.