How Soap is Made

Soap making requires two basic ingredients: lye and some type of fat. The fat can be either animal or vegetable based. Tallow, which is rendered beef fat, and lard, which comes from pigs, are common types of animal fats used in soap making. Many types of vegetable oils such as olive, palm or coconut oil can also be used. Animal and vegetable based fats and oils contain fatty acids, which are composed of trigycerides and glycerol. Fatty acids are necessary ingredients for making soap. Petroleum based oils will not work to make soap because they do not contain fatty acids. Different types of fats and oils will give different properties to soap.

Lye is a strongly alkaline (basic) substance that is extremely caustic and can cause severe burns if it comes in contact with the skin or eyes. In the past, a type of lye called potash (potassium carbonate) was made by leaching it from the ashes of hardwoods such as oak or apple. Ashes from other types of wood can also be used to produce lye, but hardwoods work best. Now commercial lye (sodium hydroxide or NaOH) is commonly used to make bar soap.

The Saponification Process

Chemically, when a base is combined with an acid, they undergo a chemical reaction and produce a salt and water. A classic example is mixing sodium hydroxide with hydrochloric acid to form sodium chloride (table salt) and water. Both the base and acid are neutralized and converted into different compounds.

The same principle holds true for making soap. When alkaline lye is combined with the fatty acids in fats or oils, a chemical reaction called saponification takes place. During this process, the lye is neutralized and soap is produced. Soap is described in chemical terms as the "alkali salt of a fatty acid." Glycerol, also called glycerin, is a byproduct of this reaction. Glycerin is a humectant, or moisturizing agent, and is beneficial to have in soap so that it is less drying to the skin. Commercial soap usually has the glycerin removed during the manufacturing process. Handmade soap retains the glycerin, so it is milder and has more moisturizing properties than commercially produced soap.

How Soap Helps to Clean Things

Soap cleans and removes soil because it is a surfactant, a word derived from "surface active." A surfactant decreases the surface tension of water so that the water is "wetter," and better able to make contact with grease or other types of soil. This then allows the soil to be loosened and dispersed, so it can be washed away.

Sources:

http://www.chemistryexplained.com/Ru-Sp/Soap.html
http://www.chandlerssoaps.com/chemistry-of-soap.html
http://en.wikipedia.org/wiki/Saponification
http://candleandsoap.about.com/od/soapmakingsafety/tp/Making-Lye-from-Wood-Ashes.htm
http://www.scienceinthebox.com/en_UK/glossary/surfactants_en.html#one