Soap is a 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.

Historically, soap has been composed of sodium (soda ash) or potassium (potash) salts of fatty acids derived by reacting fat with lye in a process known as saponification. The fats are hydrolyzed by the base, yielding glycerol and crude soap.

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

How soap works

Soaps are useful for cleaning because soap molecules attach readily to both nonpolar molecules (such as grease or oil) and polar molecules (such as water). Although grease will normally adhere to skin or clothing, the soap molecules can attach to it as a “handle” and make it easier to rinse away. Applied to a soiled surface, soapy water effectively holds particles in suspension so the whole of it can be rinsed off with clean water.

(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.