Soap is an anionic surfactant used in conjunction with water for washing and cleaning that historically comes in solid bars but also in the form of a thick liquid.

Soap, consisting of sodium (soda ash) or potassium (potash) salts of fatty acids is obtained by reacting fat with lye in a process known as saponification. The fats are hydrolyzed by the base, yielding alkali salts of fatty acids (crude soap) and glycerol.

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

How soap works

(fatty end) :CH₃-(CH₂)_n - COONa: (water soluble end)

The hydrocarbon ("fatty") portion dissolves dirt and oils, while the ionic end makes it soluble in water. Therefore, it allows water to remove normally-insoluble matter by emulsification.

Soapmaking

Handmade soap differs from industrial soap in that, usually, an excess of fat is sometimes used to consume the alkali (superfatting), and in that the glycerin is not removed leaving a naturally moisturising soap and not pure detergent. Superfatted soap, soap which contains excess fat, is more skin-friendly than industrial soap; though, if not properly formulated, it can leave users with a "greasy" feel to their skin. Often, emollients such as jojoba oil or shea butter are added 'at trace' (the point at which the saponification process is sufficiently advanced that the soap has begun to thicken), after most of the oils have saponified, so that they remain unreacted in the finished soap. Superfatting can also be accomplished through a process called superfat discount, where, instead of putting in extra fats, the soap maker puts in less lye.

Lye

Reacting fat with sodium hydroxide will produce a hard soap.

Reacting fat with potassium hydroxide will produce a soap that is either soft or liquid. Historically, the alkali used was potassium hydroxide made from the deliberate burning of vegetation such as bracken, or from wood ashes.

Fat

An array of oils and butters are used in the process such as olive, coconut, palm, cocoa butter, hemp oil and shea butter to provide different qualities. For example, olive oil provides mildness in soap; coconut oil provides lots of lather; while coconut and palm oils provide hardness. Sometimes castor oil can also be used as an ebullient. Most common, though, is a combination of coconut, palm, and olive oils.