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Re: Aquatic Plants Digest V3 #907

> From: ruddigar at home_com
> Subject: Boiling water
> I was always under the impression that boiling water would increase
> the hardness of the water (both DH and GH) because as the water is
> boiled, it evaporates.  

	Boiling water does reduce teporary (carbonate) hardness.  That is why
is is called "temporary".  Boiling drives off dissolved CO2, displacing
the equilibrium:

	2HCO3-  <->  H20 + CO2 + CO3--

to the right.  Calcium carbonate (as opposed to bicarbonate) has a very low
solubility product in water, so the CaCO3 comes out of solution.  If you
want it to _stay_ out of solution, you have to separate the water and the 
CaCO3 before the CO2 comes back after the water cools.

	The KH and GH will be reduced by the same amount, and if the
GH was > the KH to start with, I think the KH can be reduced to a very
low value.  You don't have to boil off a significant amount of the water
to do this, just enough to get rid of the CO2, which has low solubility
at high temperature.

Paul Sears        Ottawa, Canada