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Explain the formation of scum when hard water is treated with soap.

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Explain the formation of scum when hard water is treated with soap.

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Soap is basically sodium or potassium salts of higher fatty acids. Hard water contains Ca2+ and Mg2+ ions as their salts. When soap is added to hard water, the corresponding calcium and magnesium salts are formed. These are in the form of precipitates, also called scum.

Ca2+ + 2RCOONa → (RCOO)2Ca + 2Na+

(Hard water)  (Soap)    Cal. salt (ppt.)

Mg2+ + 2RCOONa → (RCOO)2Mg + 2Na+

(Hard water)  (Soap)    Mag. salt (ppt.)

where R is alkyl group having 15 to 17 carbon atoms.

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