Glycerin is an organic compound known more formally as glycerol. Its common sources are animal fat and vegetable oil. Glycerin is a clear, odorless liquid at room temperature, and it has a sweet taste. It's most commonly used in soap and is also a common ingredient in many pharmaceuticals.
Glycerin has the molecular formula C3H5(OH)3. It consists of a chain of three carbon atoms such that each carbon atom is bonded to a hydrogen atom (H+) and a hydroxyl group (OH-). Each of the two terminal carbon atoms has an additional hydrogen atom so that all three carbon atoms have a total of four bonds. Carbon has a valence of four, meaning that it has a tendency to form four bonds.
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Fatty acids are a class of compounds that are essentially a long chain of carbon atoms attached to various combinations of oxygen and hydrogen atoms. Each fatty acid molecule ends with a carbon atom that forms a double bond to an oxygen atom and a single bond to a hydroxyl group. This group has the formula COOH- and is known as a carboxyl group.
Plants and animals use glycerin and fatty acids to make triglycerides, which are the primary component of fat. The carboxyl end of each fatty acid reacts with the hydroxyl group on each carbon atom of a glycerin molecule. This results in each fatty acid molecule being bound to a carbon atom in the glycerin molecule. This process also liberates a molecule of water for each fatty acid molecule for a total of three water molecules.
Making Soap at Home
Soap is made from fats and lye. The manufacture of soap is a complex process with multiple steps, but it involves using the lye to break the fat down into sodium salts of fatty acids and glycerin. An at-home soapmaker will make the soap directly from this mixture of sodium salts of fatty acids and glycerin, and simply pour the mixture into molds.
A commercial soapmaker will wish to recover the glycerin in the soap mixture so that it can be sold separately. Salt is added on top of the soap mixture, which causes the fatty acid salts to rise to the top. The fatty acid salts are then skimmed off the top and made into commercial soap. The remaining liquid is composed mostly of glycerin. This liquid is then distilled to separate the impurities from the pure glycerin. Glycerin is commonly used as a sweetener in foods safe for people living with diabetes since it doesn't raise blood sugar levels. It's also used in the food industry as a filler and thickening agent.