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Answer -
The cleansing action of soap isdue to emulsification and micelle formation. Soaps are basically sodium andpotassium salts of long chain fatty acids, R-COO-Na+. Theend of the molecule to which the sodium is attached is polar in nature, whilethe alkyl-end is non-polar. Thus, a soap molecule contains a hydrophilic(polar) and a hydrophobic (non-polar) part.
Whensoap is added to water containing dirt, the soap molecules surround the dirtparticles in such a manner that their hydrophobic parts get attached to thedirt molecule and the hydrophilic parts point away from the dirt molecule. Thisis known as micelle formation. Thus, we can say that the polar group dissolvesin water while the non-polar group dissolves in the dirt particle. Now, asthese micelles are negatively charged, they do not coalesce and a stable emulsionis formed.