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by cellular 29 days ago
"Density is greatest at 39F, not 32F"

But density at "greatest" would mean volume shrinking...right?

I don't understand the article's logic.

1 comments

Yes, exactly. Water occupies the least volume per unit mass (equally, per unit molecule) at 39F. As it cools further, it paradoxically begins expanding in volume (as the polar ends seeking out their opposites in other molecules introduce a loose order to the liquid randomness).

Then, suddenly, at 32F/0C, water's real miracle happens. All the loose molecules click into place with other molecules - well, nearly all, because ice cubes aren't monocrystalline, typically, but those spare unmatched ends of molecules at crystal interfaces are still trapped by their other ends, and any single molecule unable to "find" a match for either strongly attractive end will still be trapped as well into solid-matrix mechanics. At most, those solo unmatched molecules become slight diffaction points for phonons (sound-energy propagation), but they are in fact so rare and so slight that they are generally undetectable. (The crystal faces are made of millions of unmatched molecule ends, and have therefore much more effect.)

Other polar molecules can act similarly, but water is astoundingly smaller than most others - so its molecules "find" their matches more easily, as they don't need to "rotate" much mass to line up - and is much less flexible as a result (so the alignment on the two-molecule level forms a more rigid body than, say, a polar sugar, and the volumetric effect is more pronounced).