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Re: If an oxygen molecule...

> The tank at the top of the water tower is at 
> atmospheric pressure, resulting in the gasses 
> reaching equilibrium levels. If the water is 
> near or super-saturated when it comes out of 
> the faucet, where does the additional gas 
> concentration originate?

Additional gas is forced into the water through

A general rule is 1 psi (pound per square inch)
per two feet of elevation (sorry, metric users.)
So, a 200 foot water tower should exert 100 psi
at the base of the tower.  Most municipalities
attempt to control water pressure at about 40 psi
(high enough to drive your sprinklers, low enough
to keep from blowing your pipe fittings.)  Of
course, as you pointed out, pressure can be
controlled through pumps, depth, restrictions,
and general laminar flow fluid dynamics where
resistance occurs at non-linear rates approaching
the sides of the pipe (hence, different pressures
exist stepping across a gradient from different 
pipe diameters.)

There are pockets of air in almost all pipes that
result from bacterial and chemical reaction, or
from turbidity in the system (stop and go flow
traffic, pressure changes, turbidity around bends
and diameter steps.)  These pockets are forced 
into the water at higher pressure, and "de-gass"
at lower pressure.  If you're a drop of water en
route to a faucet, you really have a violent ride
(if you can keep yourself together at all ;-).

Temperature changes are also important because
that affects saturation level.  Remember the old
physics lesson:  "When internal pressure equals
external pressure, a liquid boils."  Water boils
at lower temperature at higher elevation.  More 
practically, plumbers know that it's always the 
hot water pipes that freeze and bust in the winter,
because they came from the hot water heater and
cooled, they have more dissolved gas, so it takes 
a smaller energy change to freeze the pipe (more
gas, less water, gas has a lower specific heat
value for a temperature change.)

Most high rise buildings have pumps in the basement
to keep the water pressure high, if they don't have
a dedicated water tower on the roof.  Usually, the
lower floors have significantly higher pressure such
that drinking a glass of faucet water will yield a 
good belch (useful if you engage in some contests 
of social sophistication.)

expellingly yours,

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