IMAGIRYING TO live in a world dominated by dihydrogen oxide, a pound that hasno taste or smell and is so variable in its properties that it is generally benign but at othertimes swiftly lethal. Depending on its state, it scald you or freeze you. In the presence ofcertain anic molecules it form carbonic acids so nasty that they strip the leavesfrom trees ahe faces off statuary. In bulk, when agitated, it strike with a fury thatno human edifice could withstand. Even for those who have learo live with it, it is anoften murderous substance. We call it water.
Water is everywhere. A potato is 80 pert water, a cow 74 pert, a bacterium 75pert. A tomato, at 95 pert, is little but water. Even humans are 65 pert water,making us more liquid than solid by a margin of almost two to one. Water is strauff. It isformless and transparent, a we long to be beside it. It has no taste a we love thetaste of it. We will travel great distances and pay small fortuo see it in sunshine. Ahough we know it is dangerous and drowns tens of thousands of people every year, we』t wait to froli it.
Because water is so ubiquitous we tend to overlook what araordinary substa is.
Almost nothing about it be used to make reliable predis about the properties of otherliquids and vice versa. If you knew nothing of water and based your assumptions on thebehavior of pounds most chemically akin to it—hydrogen selenide or hydrogen sulphidenotably—you would expect it to boil at minus 135 degrees Fahre and to be a gas at roomtemperature.
Most liquids when chilled tract by about 10 pert. Water does too, but only down to apoint. O is within whispering distance of freezing, it begins—perversely, beguilingly,extremely improbably—to expand. By the time it is solid, it is almost a tenth morevoluminous than it was before. Because it expands, ice floats on water—「an utterly bizarreproperty,」 acc to John Gribbin. If it lacked this splendid waywardness, ice would sink,and lakes and os would freeze from the bottom up. Without surface ice to hold heat ier』s warmth would radiate away, leaving it even chillier and creati more ice.
Soohe os would freeze and almost certainly stay that way for a very long time,probably forever—hardly the ditions to nurture life. Thankfully for us, water seemsunaware of the rules of chemistry or laws of physics.
Everyone knows that water』s chemical formula is H2O, which means that it sists of onelargish oxygen atom with two smaller hydrogen atoms attached to it. The hydrogen atomsg fiercely to their oxygen host, but also make casual bonds with other water molecules.
The nature of a water molecule means that it engages in a kind of dah other watermolecules, briefly pairing and then moving on, like the ever-ging partners in a quadrille,to use Robert Kunzig』s nice phrase. A glass of water may not appear terribly lively, but everymolecule in it is ging partners billions of times a sed. That』s why water moleculesstick together to form bodies like puddles and lakes, but not so tightly that they 』t be easilyseparated as when, for instance, you dive into a pool of them. At any given moment only 15pert of them are actually toug.
In one sehe bond is very strong—it is why water molecules flow uphill whensiphoned and why water droplets on a car hood show such a singular determination to beadwith their partners. It is also why water has surface tension. The molecules at the surface areattracted more powerfully to the like molec