Sojourning unexpectedly in England while my mother goes through a bad patch, I have been reading The Guardian in between hospital visits, and I am reminded why it used to be my favourite English newspaper (even if it is a rather stiff £1.80, or over $3, a pop nowadays).
In amongst the interminable postmortems of the recent British general election was a particularly interesting article about water, and why this colourless, odourless liquid is such an important and unusual substance. It seems that water breaks many of the physical and chemical rules that most other substances obey, and it is this very rule-breaking that gives water the properties it needs to make it so flexible and useful.
For example, solid (or frozen) water floats on liquid water, unlike most other solid/liquid combinations, a phenomenon that occurs because water expands when it freezes. In this way, water seeping into rock cracks freezes and breaks up the rocks, forming soil. Floating sea and lake ice also insulates the water below it, allowing fish, plants and other organisms to survive even the harshest of winters, and ultimately allowing the development of complex life over geological periods of time, in spite of severe ice ages (which might well have done for life on Earth before we even arrived).
If water "followed the rules", all the Earth's water would exist only as water vapour, just one element of a thick, muggy atmosphere above a desiccated, inhospitable surface. Other light molecules like hydrogen sulphide, hydrogen chloride, ammonia, etc, exist as gases in the Earth's ambient conditions; water, however, is present in its liquid, gas AND solid states.
Water is composed of two hydrogen atoms and one oxygen atom. In its liquid state, the hydrogen atoms of one water molecule are strongly attracted to the oxygen atoms of other water molecules, and each water molecule can form up to four of these hydrogen bonds. This gives water molecules a cohesiveness and "stickiness" unique among liquids, and more energy than normal is needed to separate them (e.g. to boil the liquid into gas), allowing for, among other things, liquid water on the surface of the Earth. It is also this stickiness that enables such things as the movement of water through the tiniest of blood vessels, or up a plant's roots and stems. It also means that water's surface tension is much greater than that of many other liquids, which has many repercussions in the natural world.
The structure of water means that it sticks to almost anything else it comes across, making it the nearest thing we have to a universal solvent, and allowing it to easily dissolve or tear apart other compounds. Indeed, water is one of the most reactive and corrosive chemicals we know, a property that is crucial for life as we know it. It dissolves a wide variety of nutrients and other chemicals around the body, and the functioning of DNA itself relies on water's hydrogen bonds. Protein folding uses interactions with water molecules to achieve the required three-dimensional shapes.
Water has other strange and interesting properties. For example, water is at it most dense at 4ºC; due to the so-called Mpemba effect, hot water freezes faster than cold water; water is less compressible than most other liquids (even at a mile deep ocean water ia only compressed by about 1%); etc.
It really is the most fascinating and strange stuff, and I thank The Guardian for explaining some of its unique properties.
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