An additive is anything that is added to a process that is not a basic raw material. It is usually present in such small quantities that it does not interfere with final product quality. It is usually added to act as an enhancer or to prevent some unwanted reaction. For instance, anti-foam additives are added to columns, evaporators, reactors, etc. to prevent foaming. Inhibitors are added to Styrene systems to prevent polymerization. A well-known additive is a detergent added to motor oils and gasoline to keep your engine clean.