Re: Density of Water

["Adam Shaw" <adams1 at comcen_com.au>]

James Purchase was discussing the accuracy of the density of water -->
"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 inaccuracy in the metric system"

As far as I can add James, I guess you could think of it in a few ways.
How often would you ever have exactly 1.000000000 inches? (Or for the
rest of us that work by the metric system 1.00000000mm etc?). For most
of the world not operating on electron balances and such, it is rare to
get any more than 4 decimal places accuracy when talking of small units
such as grams or millimeters.

But to answer the question at hand - the quick answer is - the major
contributors to the density of water are pressure, temperature, and the
amount of ions in solution.

Now the explanation... Not detailed, but enough to suffice for the
moment I hope..

I'd say as you'd be aware, as a material heats up, the atoms gain energy
and vibrate (or molecules if a molecular compound like water is).
Inter-molecular distances become larger. Water is in the fluid state
because the molecules have more energy - with all that energy they're
not happy being close together - the inter-molecular distances grow
large enough such that the particles slide over each other. As the
temperature rises, inter molecular distances get greater and therefore
density decreases (less molecules in a given volume and a greater amount
of 'void space'). Water is usually approximated to be 1000.0kg/m3 at
5DegC. As you've more accurately found, at around 3.xx (something)
degrees the density of "pure" water is closer to your magical
1.0000g/mL. As a point of interest, the density of water at 50degC is
around 998kg/m3, and the density of water close to the transitional
temperature (100Deg) drops to around 958.5kg/m3. Also note however that
the density starts to decrease again on the other side of 5Deg due to
other molecular interactions that I won't go into (the decrease in
density can be seen through ice having larger volume than the liquid,
but containing the same amount of mass).

As you've pointed out, density of water is largely unaffected by
pressure. For the most part this is true when talking about ideal fluids
(not 'real fluids'). Water actually IS slightly compressible and it is
possible to alter the density of water through application of pressure.
It actually is possible to force water to freeze at room temperature by
applying huge pressures, and I'm sure you've seen water boil at room
temperature due to vacuum pressures. Most fluid flow calculations used
for engineering purposes (with the exception of some industry and
process plants operating with huge pressures), and a lot of scientific
calculations take water as being largely incompressible since the
pressures required to make much of a difference are quite significant.

The density of water will also change depending on the amount of
dissolved solids. Depending on scales of reference in use (the accuracy
of measurement), even small amounts of ionic solutions can alter the
density somewhat. When you get to more concentrated solutions, the
density rise (or drop) can be quite noticeable. Take salt water for
example - sea water has an SG around 1.026-1.028 on average (i.e. around
1026-1028kg/m3 at 5 degrees).

Hope this helps! Feel free to contact me off-post if you would like a
more detailed level of explanation to calm your water worries :)

Cheers,
Adam Shaw
Australia