Soaps are sodium or potassium salts of long chain fatty acid (carboxylic acid) that have cleaning action in water. Detergents are sodium salt of long chain benzene sulphonic acids.
Cleansing action of Soap : When soap is dissolved in water, it forms a colloidal suspension. In this, the soap molecules cluster together to form micelles. When a dirty cloth is dipped into a soap solution, its non-polar hydrocarbon end of micelles attach to the grease or oil present in dirt and polar end remains in water layer. The mechanical action of rubbing subsequently, dislodges the oily layer from the dirty surface shaping it into small globules, which can now be readily washed with water. Soap does not give froth easily with hard water, which contains calcium and magnesium salts. Soap first reacts with the calcium ions and magnesia ions to from insoluble precipitates of calcium and magnesium Salts of fatty acid.
Two problem arises due to the use of detergents :
(i) Detergents are non-biodegradable i.e. they cannot be decomposed by microorganisms.
(ii) Detergents have strong cleaning action.