Re: Weather Unrecovery; Photo Recovery

In article <4797ad96$1$fuzhry+tra$mr2ice@xxxxxxxxxxxxxxxx>,
Shmuel (Seymour J.) Metz <spamtrap@xxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

In <1200999567.76473.1@xxxxxxxxxxxxxxxxx>, on 01/22/2008
at 10:59 AM, Richard Gadsden <richard@xxxxxxxxxxxx> said:

CaCO3 => CaO + CO2

CaO + H2O -> Ca(OH)2.

How does that make the water softer? Heating a solution of Calcium
bicarbonate might give you a precipitate, but not Calcium Carbonate,
or at least not that I can see.

Oh, it does, and that's where 'kettle fur' and 'Boiler scale' come from.

Calcium is divalent.
Carbonate is likewise divalent.
Bicarbonate ("Hydrogen Carbonate") is monovalent.

Calcium carbonate is CaCO3
Calcium bicarbonate is Ca(HCO3)2 but only exists in solution[1].

Heating calcium bicarbonate gives you the carbonate, water, and CO2:

Ca(HCO3)2 => CaCO3 + H2O + CO2

The carbonate is best considered insoluble unless you're speaking
geologically, and precipitates out.

I suspect the bicarbonate is formed very slowly from the carbonate
in the presence of water and CO2, or from the hydroxide (this is
the standard test for CO2 in the school chemistry lab: produce
"lime water" by shaking calcium hydroxide ("slaked lime") up in water.
Bubbling CO2 through it turns the solution cloudy, due to the carbonate
being formed... <thinks>

Ca(OH)2 + CO2 => CaCO3 + H2O

....and I've just remembered that if you /continue/ bubbling CO2 through
the cloudy solution, it goes clear again, which will be due to the
conversion of the carbonate to the bicarbonate:

CaCO3 + H2O + CO2 => Ca(HCO3)2

QED

(You can demonstrate this to yourself with some "garden lime" (slaked
lime)[2], a tumbler, water, and a drinking straw - it will react to the
CO2 in your breath.)

BTW, what are the tradeoffs for Acetic versus Citric acids for cleaning?

Calcium Citrate is probably more soluble in water than Calcium Acetate.
The *best* is to use a bit of hydrochloric acid, since calcium chloride
is amazingly hygroscopic - an open jar of the solid will absorb enough
water vapour from the atmosphere to turn the whole lot into solution
(given an atmosphere with free water molecules[3] and enough time).

Chris. (I am not a chemist, some things just stick in the memory.)
[1] Evaporating the solution will convert it to the carbonate.
[2] What do you mean, you "haven't got any slaked lime"? Use some
of your BOFH supply of quicklime and add it (cautiously) to water.
[3] To deter the Very Fast Nitpicket: "at a reasonable temperature,
i.e. one which will support carbon-based organic lifeforms", OK?

--
A species willing to implement RFC1149 has already proven itself without
shame. -- David Cameron Staples in the scary devil monastery.
.

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