Water As a Solid Citizen
Even though fall this year was as gentle and gradual as any I can remember, the first freeze caught me not quite prepared for winter. I was reminded of that recently when I dug out the last jugs of rainwater saved for irrigating the garden. The garden is now only a few brown stalks poking above the snow, and the water jugs are bulging, wobble-bottomed globs of ice. They'd make good roly-poly toys for snowmen.
Why water won't fit in once perfect-size containers--whether pipes or jugs--after it freezes seems wrong somehow. Most substances contract when they change state from liquid to solid; water is one of the few that expands. I've encountered many explanations for the phenomenon, but they've never quite convinced me. The problem is that scientists speak in precise, exact terms, whereas I listen in metaphoric, relative terms. I'm always trying to relate what I don't know to something I do know, or at least can envision.
Thus I was delighted to come across a nicely metaphoric description of water's behavior. The encounter was due to my day job, so to speak; as an editor for the University of Alaska Press, I get to look over many manuscripts before nearly anyone but their authors have read them. In process right now is another work from Neil Davis, this one on permafrost and frozen ground. Naturally, it covers water's freezing.
It's hard for humans who live on the macroscale of plants, animals, and planets to think in terms of the microscale components making up these features, the molecules, atoms, and subatomic particles that dither about in a great deal of space. To help his readers grasp the situation, Davis recast an analogy first offered by Lord Kelvin: if water molecules were ships, then water in its gaseous state--water vapor--would be like ships on the high seas, so distant as to be almost always out of sight of one another. In liquid water, the molecules would be like ships packed into a harbor, closely arrayed and even tied together yet still mobile. The molecules of water in its solid state, ice, would be like ships held rigidly in drydock.
That's a vivid metaphor, but why should the drydocked ships--the ice molecules--stand farther apart than the ones in the harbor?
In his manuscript, Davis leaves metaphors for awhile to discuss the complex forces at work within aggregations of water molecules. For example, he tells readers that Van der Waals force--the relatively weak electrostatic attraction of an atom's nucleus for another atom's electron swarm--helps liquid water become more dense down to the temperature of 4 degrees centigrade (39.2 degrees F). Below that temperature, the hydrogen bonds take over: roughly put, the nuclei of each water molecule's two atoms of hydrogen, though bound to their own oxygen atom, carry a tiny positive electric charge that catches onto anything with negative charge, such as some electrons in other water molecules' oxygen atoms. The hydrogen bond, in balance with other forces at work on this scale, demands a certain space between the molecules.
I understand the concept, but I didn't quite get the picture until Davis gave me a metaphor. Imagine that the molecules in cooling water are like a mob of soldiers getting ready for an officer's review. At first the soldiers are relaxed, milling about, some standing apart, others in tight groups. But as the time for the review gets closer (or, in the case of water, as the temperature falls), the soldiers become more orderly. Finally, a command rings out (the freezing point is reached) and the soldiers push an arm's length (a hydrogen-bond distance) apart, rigidly in position, frozen into regular ranks.
Got it. Whenever I see ice floating on water, I'll remember those military molecules, standing formally apart so they take up more space than their still liquid buddies below.
