Soap

Soap is a salt of a long-chain fatty acid that acts as a cleanser and surfactant. Its molecules have water-attracting ionic heads and oil-attracting hydrocarbon tails.

Surfactant: A substance that lowers surface or interfacial tension, often by accumulating at boundaries between liquids or between a liquid and a solid. Soap is a surfactant whose two-part molecules gather at water–oil and water–surface boundaries.
Fatty acid: A carboxylic acid with a long hydrocarbon chain, found in fats and oils. Soap’s anionic portion is the carboxylate form of a fatty acid.
Saponification: The alkaline hydrolysis of fats or oils that yields glycerol and fatty-acid salts. This reaction is the traditional chemical route for making soap from fats.
Ancient Babylonian soap: Early soap-like material made from fats and alkaline substances in ancient Mesopotamia. Babylonian records attest to early recipes for fat-and-alkali mixtures.
Handwashing: The cleaning of hands with water, often using soap, to remove soil and reduce microorganisms. Soap-assisted handwashing is a standard practice for reducing transmission of infectious agents.
Micelle: A cluster of surfactant molecules arranged in a liquid, typically with hydrophobic parts inward and hydrophilic parts outward. Soap molecules can surround oily material in micelles that disperse through rinse water.
Triglyceride: A lipid consisting of glycerol esterified with three fatty acids. Animal fats and vegetable oils are largely triglycerides used as soapmaking feedstocks.
Cold-process soapmaking: A soapmaking method that combines oils and alkali without externally cooking the mixture through the reaction. It is a common way to make solid soap while retaining glycerol in the bar.
Pliny the Elder: A Roman author and naturalist whose first-century encyclopedic work described soap among other substances. His account records soap use among some peoples in the Roman world.
Ignaz Semmelweis: A Hungarian physician who showed that hand disinfection reduced childbed fever deaths in a Vienna hospital. His hospital reforms made hand cleaning central to preventing puerperal fever.

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