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Density of Water

Since this seems to be the week for off-beat discussion, maybe someone with
a better grasp of physical chemistry than I have can ante up some

In our calculations for a lot of things, we take the density of water to be
1 g per mL (or, 1 kg per L). I know that water, being a liquid is only
slightly affected by pressure, but its density does change slightly with
temperature, being densest close to 4C. (the exact temp. being either 3.97C
or 3.98C, depending upon the reference I consult).

But is is ever REALLY 1.000000 g per mL? I can find all sorts of references
which say that it gets close - really, really close, but the "closest" value
I can find for pure water is 0.9999750 g @ 3.98C @ 1 atm pressure (sea
level). One reference went so far to say that it "should" be 1 g, except for
an early calibration innacuracy in the metric system.

I realise that I'm splitting hairs here - this level of precision isn't
needed for our purposes, but its been driving me batty trying to tract down
the truth.

Does anyone know?

James Purchase