Water

A bent molecule

Water is two hydrogen atoms bonded to one oxygen atom. On paper the three might sit in a line; in fact they form a V, with the H–O–H angle at about 104.5° and each O–H bond about 96 pm long. The oxygen holds four pairs of electrons — two shared with the hydrogens, two kept to itself — and the four pairs spread out toward the corners of a tetrahedron. The two unshared pairs take slightly more room and squeeze the bonds together, from the tetrahedral 109.5° down to 104.5°.

The bend is why water behaves as it does. Oxygen pulls the shared electrons toward itself, so the oxygen end of the molecule is slightly negative and the hydrogen ends slightly positive; because the molecule is bent, the charges do not cancel, and water is strongly polar. Each molecule can bond loosely to as many as four neighbours through its hydrogens and its lone pairs — the hydrogen bond. Hydrogen sulfide, water's nearest chemical relative, is a gas that boils at −60 °C; water, held together by hydrogen bonds, stays liquid to 100 °C. The same bonds make water an unusually good solvent, give it a high heat capacity, and let it climb the inside of a narrow tube.

Why ice floats

A solid lighter than its own liquid, and why lakes freeze from the top

Almost every substance is denser as a solid than as a liquid. Water is the famous exception. When it freezes, each molecule locks into hydrogen bonds with exactly four neighbours, in an open hexagonal lattice with more empty space than the jostling liquid had. Ice at 0 °C has a density of about 0.917 g/cm³, about 8 % less than the water it floats on — it takes up about 9 % more room — which is why a bottle left in the freezer bursts and why an iceberg shows about a tenth of itself above the surface.

The effect begins before freezing. Cooled from room temperature, water contracts as expected until about 4 °C, then expands slightly as the open, ice-like structure starts to form. So the densest water in a winter lake is at 4 °C, and it sinks; the colder water rises and freezes at the top. The lid of ice insulates what lies beneath, and fish, plants and the life in the mud spend the winter in liquid water at 4 °C. If ice sank, lakes and shallow seas would freeze from the bottom up, and much of them would never thaw.

Water on Earth and in living things

Water covers about 71 % of the Earth's surface, and nearly all of it is salt. Of an estimated 1.4 billion cubic kilometres, some 97.5 % is in the oceans and other saline bodies; of the 2.5 % that is fresh, more than two thirds is locked in ice sheets and glaciers and most of the rest lies underground. The rivers and lakes that people drink from hold well under one per cent of the fresh water — about a hundredth of one per cent of the whole.

Living things are mostly water: an adult human about 60 % by mass, a jellyfish about 95 %. Almost every reaction of life happens in it. Water dissolves salts, sugars and gases and carries them through cells and blood; it takes part directly in reactions, splitting proteins and starches and being split itself in photosynthesis; and its high heat capacity keeps a body's temperature steady. Life as we know it needs liquid water, which is why the search for life elsewhere is first a search for water.

Not an element after all

Cavendish burns two airs, Lavoisier names what they make

For more than two thousand years water was an element. Thales of Miletus made it the origin of all things; Empedocles and Aristotle set it beside earth, air and fire as one of the four roots of matter, and there it stayed through the Middle Ages and into the age of the chemists.

In 1781 Henry Cavendish, a wealthy and reclusive London experimenter, sparked a mixture of “inflammable air” — hydrogen, which he had isolated in 1766 — with ordinary air in a closed glass vessel, and found the walls wet with a dew that was pure water. Burning the inflammable air with Priestley's “dephlogisticated air” gave the same. He published the work in 1784, still explaining it in the language of phlogiston.

Antoine Lavoisier heard of the experiment in June 1783, repeated it before witnesses in Paris with Laplace, and drew the conclusion Cavendish had not: water is not simple but a compound of two gases. With Meusnier he then ran the reaction the other way, passing steam through a red-hot iron gun barrel and collecting the inflammable gas that came off while the iron gained the weight of the oxygen it had taken. In 1787 he and his colleagues gave the gas its name, hydrogen, “the water-former”, and in the Traité élémentaire de chimie of 1789 water appears not among the elements but among the things made from them. James Watt had reached a similar idea in the same years, and the friends of all three men quarrelled over priority for decades. The four elements were finished either way.

Chemists rarely call it anything but water. The systematic name oxidane, admitted by IUPAC, is used only to name its derivatives.