Chemically, soap is a salt of a fatty acid. Traditionally, soap is made by the reaction between a fat and a strong alkali such as lye (sodium hydroxide), potash (potassium hydroxide), or soda ash (sodium carbonate). Historically, the alkali was leached from hardwood ashes.

The chemical reaction that yields soap is known as saponification. In the saponification of a fat to form soap the alkali and water hydrolyze the fat thus converting it into free glycerol/glycerin and soap (fatty acid salt).[1] Occasionally, saponification can occur naturally: an underground mass tomb in Sicily has corpses whose bodies are slowly becoming saponified.[2]

Many cleaning agents today are technically not soaps, but detergents, which are less expensive and easier to manufacture.

In some countries, it’s compulsory to indicate the Total Fatty Matter (TFM) content of soap that is sold to consumers, as a percentage. Usually it averages around 70%.