Originally Posted by
chem geek
Hard water is usually from magnesium and calcium, not from iron or other metals that aren't very high in concentration. Magnesium and calcium form a precipitate with soap such as magnesium or calcium stearate. This is why soap doesn't suds well in hard water and how soap scum forms. To reduce foaming in spas, one could use magnesium instead of calcium if one wanted to.
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