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Sci Tech
QUESTION CORNER
Bar of soap
Why does a bar of soap become less soapy when it nears its end?
Nidhin Chandran
Payyanur, Kerala
A soap is basically either a sodium salt or a potassium salt of long-chain carboxylic acid (fatty acid, e.g., palm oil, coconut oil, animal fat, etc, represented by R-COOH) whereas a detergent is usually a magnesium salt of long-chain sulphonic acid (represented by R-SO{-3}H).
Whether it is soap or a detergent, it is substantially a salt admixed with certain filling materials such as starch or wax along with other ingredients like perfumes for fragrance, glycerine for softness and pigments for colour.
A few more trace substances also may be there for the purpose of aesthetics and stability. The major constituent in a cake (bar) of soap is, any way, the fatty acid salt.
A salt is essentially an organized arrangement of positively charged constituents (cations) and negatively charged constituents (anions). Salts are very readily soluble in water because water is a polar solvent and embraces ions.
Even though the soap has been more or less homogeneous in its composition at the time of opening it in the bathroom, it retains some wetness at the surface after we use it along with water.
Owing to the soap’s softness at its bulk and the wetness on the surface, the ions of the fatty acid salt in the bulk are gradually drawn (migrate) towards the wet zone of the surface as days pass and get washed during usage.
Thus, the fatty acid salt part is slightly impoverished in the bulk of the soap with time. Since soapy character is due to the tfm (total fat mass), the soap becomes less soapy when it nears its end (after considerable use).
Prof. A. Ramachandraiah
Jana Vignana VedikaWarangal, Andhra Pradesh
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