It would definitely not ícombustí, since that means to chemically combine with oxygen. The hydrogen in water has already
combusted, so the question is whether the opposite happens, and the molecule falls apart.
It turns out that to answer your question, one needs another very important piece of information- how big a box the molecule is in, and what else is in the box. By itself in a very big box, the molecule will fall apart even at room temperature. Thatís because there are a lot more ways to arrange three atoms than one molecule. The hotter things are, the smaller the box needed for the molecule to fall apart and mostly stay apart. Now if many molecules are in the box, they can fall apart and then recombine with each other. The hotter the box, the more time they spend broken up. The more molecules there are per volume, the more time they spend combined as molecules.
It also doesn't make a whole lot of sense to talk about the temperature of just one molecule -- there aren't that many degrees of freedom available for kinetic energy to be put into it. A typical molecule in a very dilute gas at a very high temperature may hold together just fine, but it is moving rather quickly. For a single molecule, we can just look at it in its own frame of reference, and find out its temperature is much lower (depends on how fast it's turning and how much the bonds are jiggling I suppose). You could put the molecule in a thermal bath of something else -- say a big box with helium at 1000k plus one water molecule, and then go with the above description.
(published on 10/22/2007)