Question
Why common salt is added to precipitate out soap from the solution during its manufacture?
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Answer
Soaps are sodium salts of higher fatty acids having general formula RCOONa. On adding common salt, Na+ ion concentration increases and due to common ion effect the equilibrium (I) as shown below shifts in the backward direction leading to the precipitation of soap.
$\text{NaCl}(\text{s})\xrightarrow{\ \ \ \ \ \ \ \ \ \ }\text{Na}^+(\text{aq})+\text{Cl}^-\text{(aq)}$
$\text{RCOONa(s)}\rightleftharpoons\text{RCOO}^-(\text{aq})+\text{Na}^+(\text{aq}).$
$\text{NaCl}(\text{s})\xrightarrow{\ \ \ \ \ \ \ \ \ \ }\text{Na}^+(\text{aq})+\text{Cl}^-\text{(aq)}$
$\text{RCOONa(s)}\rightleftharpoons\text{RCOO}^-(\text{aq})+\text{Na}^+(\text{aq}).$
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