What soap is

Soap is an anionic surfactant composed of sodium or potassium salts of fatty acids.

Soap has been used as body wash and household cleaner since ancient times. Although soaps do not occur in nature (except in very rare occasions), it is quite easy to create them by using ingredients obtained from nature (natural fats and wood ashes). In the past, soap used to be made by cooking of waste animal fats and wood ashes. The development of science led to the discovery that lye is an active compound from ashes which enables transformation of fat into soap. Both sodium and potassium hydroxide (caustic soda and caustic potash) convert fats into soap, but nowadays the term “lye” usually means water solution of sodium hydroxide (NaOH). Soap consists of sodium or potassium salts of fatty acids, created by the chemical reaction (called saponification) of strong alkali (sodium or potassium hydroxide, a lye) and liquefied natural fats. Sodium hydroxide yields hard soap, while potassium hydroxide yields soft (liquid, paste or gel) soap.

Only soap obtained from natural oils or fats and lye is real soap. This also means that soap cannot be made by omitting lye. There are numerous cleaning products with excellent cleaning abilities made from synthetic surfactants, lathering agents and many other synthetic compounds that are not soaps but detergents. 

Chemical composition of handmade soap

Various soap molecules, glycerol (glycerin) and water are the main constituents of handmade soap bars. 

At chemical level, soap is not a single-compound product. Soap by itself is a mixture of sodium or potassium salts of various fatty acids. Handmade soap always contains a mix of various fatty acid salts because it is created from natural oils and fats which contain a mixture of different fatty acids. Each fatty acid influences one or more different soap properties (hardness, lathering, cleansing and conditioning). Good soap should contain a proper combination of saponified fatty acids and that is achieved by proper combination of different oils and fats. 

Handmade soap created by cold or hot (semi-boiled) process, beside soap molecules, also contains glycerin (glycerol). Glycerol is a co-product of soap-creating chemical reaction (saponification) and cannot be separated in this kind of soap-making techniques. 

The third constituent of handmade soap is water. Even after proper curing and drying, certain amount of water remains in soap as naturally occurred glycerol in soap is hygroscopic and retains water molecules.

Beside the main soap constituents, soap may contain many other compounds, usually in small quantities. Origins of these compounds are additives and unsaponifiables that naturally occur in oils.