This one is for people who want a little more details on the use of salt in soap making. Many students had trouble explaining it in the June 2010 exam. Below is the resason why addition of salt causes soap to be deposited from the reaction mixture. Solubility product is not required at CSEC, but shows up in Module 2 of Unit 1 CAPE Chemistry.
Salting out is a term often used in industry for removing salt from a solution. An example occurs in the manufacture of soap. The chief constituent of soap is sodium stearate, C17H35CO2Na – otherwise known as sodium octadecanoate. It is salted out by adding a concentrated solution of sodium chloride. This causes the product of the stearate and sodium ion concentrations to exceed the solubility product of sodium stearate and sodium ion concentrations to exceed the solubility product of sodium stearate. In this case the salting out is due to the common ion effect. The common ion is sodium, Na+.
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