The chemical reaction that produces soap is called saponification. It occurs when a hot caustic alkali solution, such as caustic soda (sodium hydroxide), acts on natural fats or oils to produce a smooth, semisolid, fatty acid salt (soap) and glycerin, or glycerol.
             There are many different procedures and techniques of making soap. Three main ones include the boiling process, the continuous process and the cold procedure,
             The boiling process is one specific procedure in which soap can easily be made at home by boiling water with animal fat or vegetable oils and wood ash. As the water boils, the fat molecules are broken down into fatty acids. Saponification occurs when these fatty acids react with potassium carbonate from the ash to form soap.
             Continuous soap making process is another common process in which the fat is split into fatty acids and glycerin in a tall column called a hydrolyser. Molten fat is continuously pumped in at the bottom of the hydrolyser and is split into fatty acids by water that is continuously pumped in at the top at high temperature and pressure. The fatty acids are taken from the top of the hydrolyser and the glycerin is drawn off at the bottom. The fatty acids are then purified in a still and neutralized with an alkali to yield neat soap.
             Cold process is anther process, which is used by small factories. In the cold method, a fat and oil mixture is agitated with an alkali solution in an open pan until the mixture thickens. It is then poured into frames and left to saponify and solidify.
             This is a diagram illustrating how milled bar soap is commercially produced in industries to form the final product that is sold. Milled bar soap is produced by means of several mixing, blending, and milling stages. Hot water and fats are mixed in a hydrolyser and the fatty acids are distilled. The mixture is then condensed and enters metering pumps where alkali is added. After the solution is mixed and blended, b...