Chemistry · Ch 15 — Chemistry in Everyday Life
Soaps
Soaps
Soaps start from animal fat or vegetable oil -- both are glyceryl esters of long-chain fatty acids. Heating these glycerides with a sodium hydroxide solution converts them into soap plus glycerol, the saponification reaction already covered earlier in the course. Common salt is added to the reaction mixture afterward specifically to lower the soap's solubility, causing it to precipitate out of the aqueous mixture so it can be recovered; the recovered soap is then finished with the desired colours, perfumes, and any medicinal ingredients.
Quality is measured by a soap's total fatty matter, or TFM value: the total amount of fatty matter that can be split back out of a sample using mineral acid. A higher TFM means a better-quality soap. The Bureau of Indian Standards grades soap on this basis -- Grade-1 requires a minimum 76% TFM, Grade-2 requires 70%, and Grade-3 requires 60% -- alongside secondary quality checks such as lather, moisture content, mushiness, and how much insoluble matter remains in alcohol. …
What this figure shows. A labelled diagram showing soap molecules oriented at a grease surface: their non-polar hydrocarbon tails dissolved into the grease, their polar carboxylate heads (each paired with a Na+ counter-ion) facing outward into the surrounding water. Dirt particles embedded in the film of grease are shown being lifted free as small emulsified grease droplets (micelles) float away from the main grease surface, carrying the trapped dirt with th …