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Re: Cleaning Glass

As has been said: silicon dioxide is *not* soluble in water, to even the
parts per billion level.

Yes, silicate ions are present in water. They don't come from silica.
Incorporation of sodium/potassium/barium oxide and/or water into the
silicate leads to formation of much less tightly connected frameworks,
which are then sufficiently tractable for water to dissolve them
slightly. Silicic acids (SiO2.nH2O) *are* soluble on the micromolar
level, but the source of the silicate ions in solution is one of these
less tightly connected silicates (present in many minerals) rather than
plain old SiO2 dissolving. The SiO2 framework is simply too tightly
connected to be at all reactive under such mild conditions.

You say these compounds can be removed from silicates in the presence
of CO2 or bicarbonates. This is true. However, glass is not a silicate.
Silicates refer to the ternary (or higher) less tightly connected
frameworks that come from salt inclusion, many of which are present in
many common minerals. Glass is SiO2, and is silica, not a silicate.
Silica can not be dissolved under the situations you describe;
SiO2 is chemically resistant to all acids except HF, although it
does dissolve slowly in molten hydroxides and molten carbonates. Unless
your lights are *really* hot, I don't think you need to worry about
that. The only halogen that attacks SiO2 is F2, forming SiF4 and O2,
both of which are gases and would wander off, not leave a deposit on
your glass.

I get condensation on my glass every night too. My glass isn't fogged
up. And why wouldn't the glass in the tank that's in contact with the
water all day fog up? After all, if the glass were slightly soluble,
it would be in dynamic equilibrium, and the redeposition would occur
in the form of foggy microcrystalline quartz rather than a nice glass.

If it really were soluble, why would it be so hard to remove? Soluble
substances come off quite easily, since a small amount of dissolution
will severely weaken the bond to the substrate and make the precipitate
flake off. This is why it's much easier to clean that disgusting pot
after you soak it in the sink overnight. (Well, that and because you
really don't feel like doing it right now.)

My best guess at what's going on is that it's not coming from the
condensed water at all, but from splashed water. Something in your
water is then depositing as the splashed water evaporates, and this
mystery something is then undergoing a chemical transformation that
makes it irremovable. Some (moderately) plausible explanations are
that a transition metal salt is being oxidized in air (with heat? how
hot are your lights?) or that a carbonate is decomposing to an 
insoluble oxide with evolution of CO2, which happens easily with heat
and is actually one of the best ways of making oxides.

Of course, none of this helps you get the damn stuff off your glass.

Eric Tulsky