From: glhurst@onr.com (Gerald L. Hurst)
Newsgroups: sci.chem
Subject: Re: calcium carbonate in iron pipes question
Date: 21 Dec 1995 07:29:10 GMT
Organization: Consulting Chemist
Lines: 32

In article <dAO2wkOJGzxX084yn@world.std.com>, moroney@world.std.com
(Michael Moroney) says:

>In article <4b7m1n$lea@news.asu.edu>,
>bridges@aztec.asu.edu (HARRY MOREL) wrote:
>> 
>> Why does a white substance (calcium carbonate?) deposit on the 
>> inside of iron pipes over the years?  Why doesn*t the calcium, (or 
>> magnesium) carbonate remain dissolved in the water?
>
>If you're talking about hot water pipes, calcium carbonate, unlike most
>chemicals, is less soluable in hot water than it is in cold.
>
>There may also be a reaction where calcium bicarbonate reacts to form
>calcium carbonate, CO2 and water, the CO2 either escapes or reacts with
>iron to form its carbonate, and the calcium carbonate precipitates.
>I am unsure about this last part.

I seem to recall it being calcium sulfate which has decreasing
solubility with rising temperature, at least for high water
temperatures.

Also, I believe that the observed modest solubility of calcium 
carbonate is mostly due to the formation of bicarbonate by
CO2 in equilibrium with air. Upon heating, the CO2 concentration
is lowered with the result that calcium carbonate precipitates
giving the illusion of a negative temperature coefficient of
solubility.

Jerry (Ico)

*Experience tends to make me very suspicious of Gerald's assertions :-)*
--Bruce Hamilton