Water has long been known to exhibit many physical properties that distinguish it from other small molecules of comparable mass. Chemists refer to these as the "anomalous" properties of water, but they are by no means mysterious; all are entirely predictable consequences of the way the size and nuclear charge of the oxygen atom conspire to distort the electronic charge clouds of the atoms of other elements when these are chemically bonded to the oxygen.

A covalent chemical bond consists of two atoms that share a pair of electrons between them. In the water molecule H2O, the single electron of each H is shared with one of the six outer-shell electrons of the oxygen, leaving four electrons which are organized into two non-bonding pairs. Thus the oxygen atom is surrounded by four electron pairs that would ordinarily tend to arrange themselves as far from each other as possible in order to minimize repulsions between these clouds of negative charge. This would ordinarly result in a tetrahedral geometry in which the angle between electron pairs (and therefore the H-O-H bond angle) is 109°. However, because the two non-bonding pairs remain closer to the oxygen atom, these exert a stronger repulsion against the two covalent bonding pairs, effectively pushing the two hydrogen atoms closer together. The result is a distorted tetrahedral arrangement in which the H—O—H angle is 104.5°.
					Computer-generated image of a water molecule
		These two computer-generated images of the H2O molecule come from calculations that model the electron distribution in molecules. The outer envelopes show the effective "surface" of the molecule.

The H2O molecule is electrically neutral, but the positive and negative charges are not distributed uniformly. This is shown clearly in the two images above, and in the schematic diagram at the left. The electronic (negative) charge is concentrated at the oxygen end of the molecule, owing partly to the nonbonding electrons (solid blue circles), and to oxygen's high nuclear charge. This charge displacement constitutes an electric dipole, represented by the arrow at the bottom; you can think of this dipole as the electrical "image" of a water molecule.

Liquid and solid water

	Ice we know all about: each water molecule is surrounded by four neighboring H2Os. two of these are hydrogen-bonded to the oxygen atom on the central H2O molecule, and each of the two hydrogen atoms is similarly bonded to another neighboring H2O. The hydrogen bonds are represented by the dashed lines in this 2-dimensional schematic diagram. In reality, the four bonds from each O atom point toward the four corners of a tetrahedron centered on the O atom. This basic assembly repeats itself in three dimensions to build the ice crystal.
	When ice melts to form liquid water, the uniform three-dimensional tetrahedral organization of the solid breaks down as thermal motions disrupt, distort, and occasionally break hydrogen bonds. The methods used to determine the positions of molecules in a solid do not work with liquids, so there is no unambiguous way of determining the detailed structure of water. The illustration here is probably typical of the arrangement of neighbors around any particular H2O molecule, but very little is known about the extent to which an arrangement like this gets propagated to more distant molecules.
			Here are three-dimensional views of a typical local structure of liquid water (right) and of ice (left). Notice how the greater openness of the ice structure which is necessary to ensure the strongest degree of hydrogen bonding in a uniform, extended crystal lattice.

 

The anomalous properties of water

The other widely-cited anomalous property of water is its high boiling point. As this graph shows, a molecule as light as H2O "should" boil at around –90°C; that is, it should exist in the world as a gas rather than a liquid, if H-bonding were not present. Notice that H-bonding is also observed with fluorine and nitrogen.

Water clusters, structured water and biowater

Since the 1930s, chemists have described water as an "associated" liquid, meaning that hydrogen-bonding attractions between H₂O create loosely-linked aggregates. Because the strength of a hydrogen bond is comparable to the average thermal energy at ordinary temperatures, these bonds are disrupted by thermal motions almost as quickly as they form. Theoretical studies have shown that certain specific cyclic arrangements ("clusters") of 3, 4, and 5 H₂O molecules are especially stable, as is a three-dimensional hexamer (6 molecules) that has a cage-like form. But even the most stable of these clusters will flicker out of existence after only about 10 picoseconds. It must be emphasized that no clustered unit or arrangement has ever been isolated or identified in pure bulk liquid water.

Liquid water can be thought of as a seething mass of water molecules in which hydrogen-bonded clusters are continually forming, breaking apart, and re-forming. Theoretical models suggest that the average cluster may encompass as many as 90 H2O molecules at 0°C, so that very cold water can be thought of as a collection of ever-changing ice-like structures. At 70° C, the average cluster size is probably no greater than about 25.

So-called "structured water"

Water molecules interact strongly with non-hydrogen bonding species as well. A particularly strong interaction occurs when an ionic substance such as sodium chloride (ordinary salt) dissolves in water. Owing to its high polarity, the H₂O molecules closest to the dissolved ion are strongly attached to it, forming what is known as the primary hydration shell. Positively-charged ions such as Na⁺ attract the negative (oxygen) ends of the H₂O molecules, as shown in the diagram below. The ordered structure within the primary shell creates, through hydrogen-bonding, a region in which the surrounding waters are also somewhat ordered; this is the outer hydration shell, or cybotactic region.

Biowater

Water can hydrogen-bond not only to itself, but also to any other molecules that have -OH or -NH₂ units hanging off of them. This includes simple molecules such as alcohols, surfaces such as glass, and macromolecules such as proteins. The biological activity of proteins (of which enzymes are an important subset) is critically dependent not only on their composition but also on the way these huge molecules are folded; this folding involves hydrogen-bonded interactions with water, and also between different parts of the molecule itself. Anything that disrupts these intramolecular hydrogen bonds will denature the protein and destroy its biological activity. This is essentially what happens when you boil an egg; the bonds that hold the egg white protein in its compact folded arrangement break apart so that the molecules unfold into a tangled, insoluble mass which, like Humpty Dumpty, cannot be cannot be restored to their original forms. Note that hydrogen-bonding need not always involve water; thus the two parts of the DNA double helix are held together by H—N—H hydrogen bonds.

It is now known that the intracellular water very close to any membrane or organelle (sometimes called vicinal water) is organized very differently from bulk water, and that this structured water plays a significant role in governing the shape (and thus biological activity) of large folded biopolymers. It is important to bear in mind, however, that the structure of the water in these regions is imposed solely by the geometry of the surrounding hydrogen bonding sites.

"Clustered", "Unclustered" and other structure-altered waters

The "alternative" health market is full of goofy products which purport to alter the structure of water by stabilizing groups of H₂O molecules into permanent clusters of 4-8 molecules, or alternatively, to break up what they claim are the larger clusters (usually 10-15 molecules) that they say normally exist in water. The object in either case is to promote the flow of water into the body's cells ("cellular hydration"). This is of course utter nonsense; there is no credible scientific evidence for any of these claims, many of which verge on the bizarre. There are even some scientifically absurd U.S. Patents for the manufacture of a "clustered" form of water. At least 20 manufacturers offer nostrums of this kind to the scientifically-naive public through hundreds of Web sites and late-night radio "infomercials". None of these claims is supported by credible evidence.

How chemists think about water

Does water have "memory"?

According to modern-day proponents of homeopathy, it must. Homeopathic remedys are made by diluting solutions of various substances so greatly that not even a single molecule of the active substance can be expected to be present in the final medication. Now that even the homeopaths have come to accept this fact, they explain that the water somehow retains the "imprint" or "memory" of the original solute.

In 1985, Jacques Benveniste, a French biologist, conducted experiments that purported to show that a certain type of cellular immune response could be brought about by an anti-immunoglobulin agent that had been diluted to such an extent that it is highly unlikely that even one molecule of this agent remained in the aqueous solution. He interpreted this to indicate that water could somehow retain an impression, or "memory", of a solute that had been diluted out of existence. This result was immediately taken by believers in homeopathy as justification for their belief that similarly diluted remedies could be effective as alternative medical agents. The consensus among chemists is that any temporary disruption of the water structure by a dissolved agent would disappear within a fraction of a nanosecond after its removal by dilution, owing to the vigorous thermal motions of the water molecules. Benveniste's results have never been replicated by other scientists, and they seemed so bizarre that his scientific reputation and research career were largely destroyed. In early 2001, however, an independent group of researchers, in a very carefully designed double-blind study, did report statistically significant evidence of a similar kind of "water memory" effect. A journalistic account of this work can be seen here. Only time will tell whether there is anything to this, but if there should be, then we chemists, who think we know so much about water, will once again be humbled.

References

The mystery, art and science of water. This site provides a wonderful view of water in all the many ways it impacts upon the multiple facets of our culture. Highly recommended.

Water Structure and Propertiesis a Web site developed by Martin Chaplin at South Bank University in England. It is a scientifically sound, well laid-out collection of articles on water and its structure which should answer any of your questions.

Water Clusters. K. Liu, J.D. Cruzan, and R.J. Saykally. Science 1996 929-993 - A summary of experimental data on the structures, energetics and dynamics of small clusters, and comparisons with theoretical predictions.

"Water Buckyballs": Chemical, catalytic and cosmic implications. This rather technical paper by Keith Johnson of MIT explores the quantum theory and far-i.r. spectra of water clusters and speculates on their role in cosmochemistry.

Cell-associated water. W. Drost-Hansen, J. Clegg, ed. Academic Press, 1979. This is a collection of 11 papers given at a conference in 1976. It predates the availability of modern laser-based methods of examining water structure, but contains a lot of indirect observations and NMR results.

Why is water blue? It's all about O-H bond stretching! A more technical site.

And finally, for a darker view of water, see the Ban DHMO page.