Salting out is adding sodium chloride to the soap mixture to lower the solubility of the soap so it separates and floats to the surface.
Salting out is the step of adding sodium chloride to the soap mixture to lower the solubility of the soap so that it separates and floats to the surface. Malay: penggaraman keluar · Chinese: 盐析.
After saponification, the beaker holds soap, glycerol, water and any leftover alkali all mixed together. Adding common salt raises the concentration of sodium ions in the water and makes the mixture too crowded for the soap to stay dissolved. The soap is pushed out of solution and collects as a solid curd on top, because it is less dense than the salty liquid below. The glycerol and the excess alkali stay dissolved in the water, so salting out both separates the soap and begins to purify it, leaving the by-products behind in the liquid layer.
Example. In the school preparation of soap, once the oil has been boiled with sodium hydroxide, a spatula of sodium chloride is stirred in. A curd of soap rises to the surface, is filtered off, washed with a little cold water and pressed into a bar, while the liquid below is kept because it contains the glycerol. The same principle is used on a factory scale to harvest and purify the soap.
Confusion to avoid. Do not think the salt takes part in the chemical reaction. Salting out is a physical change: it lowers the solubility of the soap and separates the soap that saponification has already made. The reaction that forms the soap is saponification; salting out only collects and cleans it.
Salting out is a standard practical step in soap-making, and describing why the soap floats after salt is added is a common SPM Chemistry short-answer point in this chapter.
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