Why Is Water Such a Strange Liquid? The Anomalies That Make Life Possible

Water is two hydrogens attached to an oxygen, and two things about the arrangement produce everything else. First, oxygen pulls electrons much harder than hydrogen does, so the electrons spend more time near the oxygen, leaving it slightly negative and the hydrogens slightly positive. Second, the molecule is bent rather than straight, at an angle of about 104.5 degrees, because the oxygen also carries two pairs of unshared electrons that push the hydrogens together. A straight molecule with the same charge separation would have its poles cancel; a bent one does not, so the molecule as a whole has a positive end and a negative end. The consequence is the hydrogen bond, an attraction between the positive hydrogen of one molecule and the negative oxygen of another, which is around a tenth the strength of the bonds holding the molecule together and which is the source of nearly every anomaly. Each molecule can form four such bonds, two through its hydrogens and two through its unshared pairs, producing a constantly rearranging three-dimensional network.

The network explains a long list of properties that would otherwise look unconnected:

• •A very high boiling point for its molecular size, because separating molecules into a gas requires breaking many hydrogen bonds
• •A high heat capacity, so water absorbs a great deal of energy for a small temperature rise, which stabilises the climate of coastal regions and the temperature of any organism made mostly of it
• •A high heat of vaporisation, which is why sweating and transpiration cool so effectively
• •High surface tension, the highest of any common liquid except mercury, which lets insects walk on ponds and drives capillary action up the stems of plants
• •The ability to dissolve an exceptional range of substances, because the charged ends surround and separate ions, which is why it is called the universal solvent and why it carries nutrients through every living thing
• •A maximum density at 4 degrees rather than at freezing, and expansion on freezing, which is why ice floats

Almost every substance is denser as a solid than as a liquid, because cooling slows the molecules and they pack closer. Water reverses this over the last four degrees. As it cools below 4 degrees, the hydrogen bonds increasingly lock molecules into the open, tetrahedral arrangement that ice requires, and that arrangement contains more empty space than the jostling liquid does, so the substance expands by about nine percent on freezing. The consequences are ecological. A lake cools from the top, and once the surface layer reaches 4 degrees it stops sinking, so the coldest water stays on top and freezes there, forming a floating insulating lid under which the rest of the lake stays liquid at around 4 degrees. Fish and everything else survive the winter. If ice sank, lakes and eventually oceans would freeze from the bottom up and would not fully thaw, and the history of life would have been very different. The same expansion is what splits water pipes and, over geological time, shatters rock.

Biology is built on these properties rather than merely tolerating them. Proteins fold into their working shapes largely because their water-repelling parts are pushed together by the surrounding water reorganising itself, an effect called the hydrophobic interaction that is driven by water's behaviour rather than by any attraction between the protein's parts. Cell membranes assemble spontaneously for the same reason. The high heat capacity buffers organisms and oceans against temperature swings. Capillary action and cohesion pull water from a tree's roots to leaves a hundred metres up, in unbroken columns under tension, with no pump involved. The solvent power carries every nutrient, waste product and signalling molecule. And the fact that water is a liquid across a wide temperature range at ordinary pressures is what gives life a working medium at all, which is why the search for life elsewhere is in practice a search for liquid water.

Water is not a solved problem. The detailed structure of the liquid, meaning how the hydrogen bond network is arranged and how quickly it rearranges, has been contested for decades, with competing models proposing either a continuous distribution of distorted bonds or a fluctuating mixture of two distinct local structures. A hypothesis that supercooled water has a second critical point, below which it separates into two distinct liquid phases of different density, has driven a large experimental effort, since the region in question freezes almost instantly and is extremely hard to probe; experiments using laser heating on microsecond timescales have produced support for it. Supercooled water can be held liquid to around minus forty degrees before it freezes spontaneously. And a number of long-standing claims, including the Mpemba effect in which hot water is said to freeze faster than cold, remain unresolved and disputed even as to whether the phenomenon is real.

Water's oddities nearly all follow from one feature: a bent molecule with a positive and a negative end, letting each molecule form up to four hydrogen bonds with its neighbours. That network raises the boiling point far above what the molecule's size predicts, gives a high heat capacity and surface tension, makes it an exceptional solvent, and forces expansion on freezing so ice floats and lakes freeze from the top. Protein folding, membrane assembly and the rise of sap up tall trees all depend on these properties, and the detailed structure of the liquid remains under debate.