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Carbon and Its Compounds
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09/09/2021 1:37 pm
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Explain the formation of scum when hard water is treated with soap.
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09/09/2021 1:40 pm
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.