What is hard water and why doesn't soap dissolve in it very well?
(Lansing State Journal, August 16, 1995)

Hard water is water that has positively charged calcium, magnesium and iron ions dissolved in it.  Any solvent such as water can dissolve only so much of any substance.  For instance, if we add salt to water, it will dissolve very easily at first.  However, the more salt we add, the harder it will be for it to dissolve.  Eventually, the water will reach a point where it can't dissolve more salt.

Soap has negatively and positively charged ions.  Both the negatively and positively charged ions have to go into solution for the soap to dissolve.  Because hard water already has positively charged ions, it has less of a tendency to dissolve any more positively charged ions compared to water that is not hard.  The positive ions in the soap can't go into solution very easily, so the soap doesn't dissolve as well as it would in normal water.

Water softeners remove positively charged ions from water so it can perform normally.

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