The Joy of Water
P.W.Atkins

There is much joy to be had from water. It is not merely its awesome abundance and the variety of its forms that are so joy inspiring, or even its essential role in the carving of our planet and the evolution of life. For me, the joy is that such rich properties can emerge from such a simple structure. Moreover, it is not merely the richness of the simplicity of water that is such an inspiration, for I also find deep satisfaction in the subtlety of its properties. That these unusual properties are crucial to the emergence and persistence of life is a dimension added to the pleasure of beholding water.

Water-be it the rolling Pacific Ocean or a droplet of morning mist, a craggy glacier of a snowflake, a gas pulsing through the blades of a steam turbine or hanging in the air as a major contribution to the global turmoil we call the weather-is composed of water molecules. Every water molecule in the world, and wherever else it occurs in the universe, is identical. Each one consists of a central oxygen atom to which are attached two hydrogen atoms. That is all. Oceans, life, and romance all stem from this simple picture.

To see the potential for the formation of the Pacific Ocean in this minuscule entity, we need to know some details about hydrogen, the lightest element of all, and its companion element, oxygen. Atoms of hydrogen are very small; they consist of a single positively charged proton for a nucleus, and that proton is surrounded by a single electron. One advantage of being small is that the nucleus of such an atom can penetrate close to the electrons of other atoms. Hydrogen nuclei can wriggle into regions that bigger atoms cannot reach. Moreover, because there is only one electron in a hydrogen atom, the bright positive charge of the nucleus can blaze through the cloudlike electron's misty negative charge, and there is, consequently, a strong attraction for other electrons that happen to be nearby.

As for the oxygen atom, it is much bigger than a hydrogen atom. Nevertheless, as atoms go, it is still quite small compared to those of other elements, such as sulfur, chlorine, and even carbon and nitrogen. Its smallness stems from the strong positive charge of the oxygen nucleus, which draws its electron close to itself. Additionally, because it is so small and yet has a strongly charged nucleus, an oxygen atom can draw toward itself the electrons of other atoms. In particular, it can draw in the electrons of any atoms to which it is bonded.

In water, an oxygen atom is bonded to two small hydrogen atoms. The central oxygen atom sucks in the electrons of the oxygen-hydrogen links, and so partially denudes the hydrogen still further of their electrons. The oxygen atom thereby becomes oxygen rich and the hydrogen atoms become electron poor. Consequently, the oxygen atom has a vestigial negative charge (arising from its being bloated with electrons) and the hydrogen atoms acquire a vestigial positive charge because the positive charge of the nucleus is no longer canceled by the surrounding electrons (for they have been partially sucked away). The resulting distribution of charge-oxygen negative and hydrogen positive-coupled with the small size of the hydrogen atoms, is at the root of water's extraordinary properties.

Another feature that conspires with the distribution of electrons and results in oceans is the shape of the water molecule. It is an angular, open-V-shaped molecule, with the oxygen atom at the apex of the V. The important feature of this shape, which can be rationalized by examining how electrons are arranged around the central oxygen atom, is that one side of the oxygen atom is exposed, and that exposed side is rich in electrons.

Now we shall see how these features bubble out into the real world of phenomena and tangible properties. Most important of all is the ability of one water molecule to stick to another water molecule. The electron-rich region of the oxygen atom is the site of negative electric charge; the partially denuded hydrogen atom of a neighboring molecule is the site of positive charge, and the opposite charges attract one another. The special link between the two water molecules mediated by a hydrogen atom in this way is called a hydrogen bond. It is one of the most important intermolecular links in the world, for its effects range from the operation of the genetic code (the two strands of the DNA double helix are linked together by hydrogen bonds), through the toughness of wood (for the ribbons of cellulose are clamped rigidly together face-to-face by the sturdy and numerous hydrogen bonds between them), and-the point of our concern-with the properties of water. For a water molecule is so light that if it were not for the hydrogen bonds that can form between its molecules, then water would be a gas, and instead of puddles, lakes, and oceans of precious liquid, there would be a humid sky full of gaseous water and barren ground beneath.

Just as hydrogen bonds between water molecules trap them into forming a liquid even at warm everyday temperatures, so they also help to form the rigid solid ice at only slightly lower temperatures. However, when ice forms from liquid water, something rather odd happens, an oddness that is also life preserving. When the temperature is lowered, the water molecules of a liquid are shaken and jostled less vigorously, and hydrogen bonds can form more extensively and survive for longer. As a result, the molecules cease flowing readily as a liquid, and a stable solid forms instead. Now the shape of the molecule comes into play. An oxygen atom in the V-shaped water molecule has room to accommodate two hydrogen bonds, one to each of two neighboring molecules. Each oxygen atom now participates in four bonds-two ordinary oxygen-hydrogen bonds, and two hydrogen bonds to neighbors-and these four bonds point toward the corners of a tetrahedron. This arrangement, which is continued neighbor after neighbor through the solid, results in a very open structure for ice, and the water molecules are held apart as well as held together, like an open scaffold of atoms and bonds. When ice melts, this open structure collapses and forms a denser liquid. When water freezes, the collapsed structure of the liquid unfurls and expands into an open structure.

In other words, almost uniquely among substances, the solid form (ice) is less dense than the liquid form. One consequence of this peculiarity is that ice forms and floats on the surface of lakes. This feature is life preserving, because the film of ice helps to protect the water below from the freezing effect of the air above, and marine life can survive and flourish even though the temperature is low enough to freeze the surface layers of water.

The hydrogen bonds in water and the tightness with which they bind molecule to molecule are also responsible for other features of water. The color of water in bulk, which gives our planet its singular hue, can be traced to them; so can the film that forms on the surface of the liquid and which curves it into droplets. Water's considerable heat capacity (its ability to store energy supplied as heat) is another consequence of these bonds, and this characteristic is put to use in domestic central heating systems, where a little water can be used to pump a great deal of energy around a house.

Another extraordinary feature of water is its ability to dissolve so much. This characteristic also stems from the peculiar arrangement of electric charges and atoms in a water molecule. Many compounds consist of ions, or electrically charged atoms. Common salt, sodium chloride, for instance, consists of positively charged sodium ions and negatively charged chloride ions. In the solid, each positive ion is surrounded by negative ions, and each negative ion is surrounded by positive ions. Water, though, with its system of positive and negative charges, can emulate both types of these surrounding ions. Thus, when exposed to water, the sodium ions of a crystal can become surrounded by water molecules that present their negatively charged oxygen atoms toward them, and thereby emulate chloride ions. Similarly, chloride ions can become surrounded by positively charged hydrogen atoms of the water molecules, which emulate the effect of sodium ions in the original crystal. Each type of ion is seduced; the sodium ions float off surrounded by water molecules emulating chloride ions, and the chloride ions float off surrounded by water molecules using their hydrogen atoms to emulate sodium ions. Water has a peculiarly strong ability to act in this way, which is why it is such a good solvent (or, in some circumstances, when it corrodes, a highly dangerous chemical). It is this ability of water that carves landscapes from stone. It transports nutrients through the soil and brings them into plants. Water pervades our bodies, and through its ability to support the free motion of ions and other molecules that it dissolves, provides an environment for life.

Water is truly a remarkable substance; so slight in structure, yet so huge in physical and chemical stature. That so meager an entity can behave so grandly is a microcosm of modern science, which seeks giants of simplicity and thereby adds joy to our appreciation of this wonderful world.

PETER ATKINS has been a Fellow of Lincoln College, Oxford University; university lecturer in physical chemistry since 1965; and visiting professor in a number of institutions, including universities in France, Japan, China, New Zealand, and Israel. He was awarded the Meldola Medal of the Royal Society of Chemistry in 1969, and an honorary degree from the University of Utrecht for his contributions to chemistry. His books include Physical Chemistry; Inorganic Chemistry; and General Chemistry. In addition, he has written a number of books on science for the general public, including The Second Law; Molecules; and Atoms, Electrons, and Change. His interests extend to cosmology and the deep contribution of science to culture. His overriding focus is on the communication of science, and he seeks to share the thrill and pleasure that scientific insights provide.

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