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Soaps and Detergents

Take some castor oil in a beaker. Add the same amount of concentrated sodium hydroxide solution to it. Heat the mixture. Stir it. A paste of soap is formed. When the reaction is complete, add NaCl in small amount and keep stirring. What happens? The soap precipitates and floats on the surface.
When soap is formed in the above activity some sodium chloride is added to it. This is called salting out of soap. Salting involves a common ion effect. The pure soap is further washed with water to remove the lye impurities. It is then transferred to crushing machine for thorough mixing. The uniform paste is then poured into moulds and allowed to solidify in big blocks. The big blocks are then cut into small cakes.
The liquid left after the preparation of soap is called spent lye and glycerol is removed from it as a by-product.


Soaps are sodium or potassium salts of higher fatty acids like stearic acid, palmitic acid, oleic acid etc. Soap is conveniently made by stirring a liquid oil or melted fat with a calculated quantity of caustic soda or caustic potash solution, which is called lye. The oils and fats are glycerides of the higher fatty acids and are saponified by the alkali to form the sodium salts of the acids (soaps) and glycerol.



Take two beakers A and B containing hard water. Pour a test tube of soap solution to beaker A and detergent to beaker B. Shake both the test tubes. What happens in beaker A and beaker B? A dirty white curd like scum is formed on the surface of water in beaker A and in beaker B. In beaker B large amount of foam is formed.
In the above activity we found that detergents lather even in hard water. This is because they do not form insoluble precipitates with the calcium and magnesium ion is hard water. They are generally ammonium or sulphonate salts of long chain carboxylic acids.

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