Question
Why does hard water not form lather with soap?
Solution
Soap is sodium or potassium salt of certain long chain fatty acids such as stearic acid, palmitic acid, oleic acid etc. When hard water is treated with soap solution, Ca2+and Mg2+ ions present in hard water react with the anions of fatty acids present in the soap to form a scum (curdy white precipitate).
Soap will not produce a lather with hard water until all the calcium and magnesium ions have been precipitated as stearates. Hard water thus wastes soap.
