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I heard something recently in a casual discussion, but have yet to be able to confirm it: is there any evidence that the bonding angle for a water molecule, currently defined as 104.5, has been either steadily growing or shrinking since it was first observed and measured? Is there any possibility that water samples from different locales would produce different results? I am speaking strictly of water in as pure a form as possible, not water affected by impurities or physical manipulation.

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Strongly related: – dmckee Feb 13 '13 at 22:55

No, there has been no change to the bond angle of pure water,1 nor does anyone expect to measure one. The reason is this angle is set by fundamental physical constants - things like the mass of the proton and the speed of light - albeit in a complicated way. Such things are called constants for a reason. Now there are some theories out there, a number of them a bit far-fetched, that speculate that maybe some of our constants have been evolving extremely slowly over billions of years, but certainly there has been no detected variation over the course of human scientific inquiry. Such a discovery would be huge, causing much agitation in all of physics and chemistry.

It should also be noted that the bond angle can both be calculated from theory and measured from experiment, and neither method yields exactly $104.5^\circ$. Now your theoretical calculations might improve if you make fewer simplifying assumptions than your predecessors, and your measurements might improve if you develop better apparatus and technique, but the true underlying value of the bond angle is not changing.

1 Defined here as oxygen-16 bonded to two atoms of hydrogen-1. Now the angle could vary with isotopic abundances, so perhaps someone was referring to changing isotopic abundances of a source of water. You might see this, for instance, in extreme climate change, since as temperatures rise the fraction of "heavy" water ($\mathrm{D}_2\mathrm{O}$, etc.) evaporated from oceans increases relative to "normal" water.

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This is really just a footnote to Chris' answer, but any change in the bond angle of water would change its rotational and vibrational spectra. We can detect the spectrum of water molecules in the Orion nebula, which is 1,344 light years away, so we know the bond angle hasn't changed in the last 1,344 years.

Presumably we can detect water in more distant nebulae, but no change for a thousand years seems a fairly definitive test in this context :-)

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And in case 1300 ly isn't enough, we can easily go out tens of millions of light years – Chris White Mar 31 '13 at 7:10

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