An industry whose products result from (1) one or more chemical or physicochemical changes; (2) extraction, separation, or purification of a natural product with or without chemical reactions; (3) preparation of specifically formulated mixtures of materials, either natural or synthetic. Examples are as follows (with allowance for some overlapping): (1) the plastics, rubber, leather, food, dye, and synthetic organic industries; (2) the petroleum, paper, textile, and perfume industries. (3) Many of these involve one or more unit operations of chemical engineering, as well as such basic processes as polymerization, oxidation, reduction, hydrogenation, etc., usually with the aid of a catalyst. This definition may be interpreted to include ore processing, separation, and refinement, as well as the manufacture of metal products; however, these are usually considered to comprise the metal and metallurgical industries.