Why does hard water make soap "scum" ?

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Offline chris

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Why does hard water make soap "scum" ?
« on: 28/12/2006 04:23:00 »
Why does hard water cause a layer of scum to form on the surface following the addition of soap?
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Offline eric l

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Re: Why does hard water make soap "scum" ?
« Reply #1 on: 28/12/2006 08:51:11 »
Water is "hard" because it contains soluble salts of polyvalent cations, mainly Ca and Mg, but also Cu and Fe, mainly chlorides, sulphates , phosphates and bicarbonates. 
(As a sideline :  bicarbonates will give carbonates + carbondioxide when cooked, that's why they give "temporarely hardness")
Traditional soaps are composed mainly of potassium- and sodium salts of fatty acids.  The hardness ions react with the fatty acids and give insoluble compounds.  Due to the effect of the still dissolved soap, the insoluble compounds end up in the foam and build the scum.
"Wonder is no wonder" (Simon Stevin 1548-1620)