chemical industry companies statistics from different countries may have different bases of calculation; indeed the basis may change from time to time in the same country. An additional source of confusion is that in some cases the production is quoted not in tons of the product itself but in tons of the content of the important component.

For purposes of simplicity, various divisions of the chemical industry companies, such as heavy inorganic and organic chemicals and various families of end products, will be described in turn and separately, although it should be borne in mind that they interact constantly. The first division to be discussed is the heavy inorganic chemicals, starting at the historical beginning of the chemical industry companies with the Leblanc process. The terms heavy chemical industry companies and light chemical industry, however, are not precisely exclusive, because numerous operations fall somewhere between the two classes. The two classes do, however, at their extremes correlate with other differences. For example, the appearance of two kinds of plants is characteristically different. The large-scale chemical plant is characterized by large pieces of equipment of odd shapes and sizes standing immobile and independent of one another. Long rows of distilling columns are prominent, but, because the material being processed is normally confined in pipes or vessels, no very discernible activity takes place. Few personnel are in evidence.

The light chemical industry companies is entirely different. It involves many different pieces of equipment of moderate size, often of stainless steel or lined with glass or enamel. This equipment is housed in buildings like those for, say, assembling light machinery. Numerous personnel are present. Both types of plant require large amounts of capital.

Because calcium sulfide contained in the black ash had a highly unpleasant odour, methods were developed to remove it by recovering the sulfur, thereby providing at least part of the raw material for the sulfuric acid required in the first part of the process. Thus the Leblanc process demonstrated, at the very beginning, the typical ability of the chemical industry to develop new processes and new products, and often in so doing to turn a liability into an asset.
Later in the 19th century the development of electrical power generation made possible the electrochemical industry. This not clearly identifiable branch of the chemical industry includes a number of applications in which electrolysis, the breaking down of a compound in solution into its elements by means of an electric current, is used to bring about a chemical change. Electrolysis of sodium chloride can lead to chlorine and either sodium hydroxide (if the NaCl was in solution) or metallic sodium (if the NaCl was fused). Sodium hydroxide, an alkali like sodium carbonate, in some cases competes with it for the same applications, and in any case the two are interconvertible by rather simple processes. Sodium chloride can be made into an alkali by either of the two processes, the difference between them being that the ammonia-soda process gives the chlorine in the form of calcium chloride, a compound of small economic value, while the electrolytic processes produce elemental chlorine, which has nearly innumerable uses in the chemical industry, including the manufacture of plastic polyvinyl chloride, the plastic material produced in the largest volume. For this reason the ammonia-soda process, having displaced the Leblanc process, has found itself being displaced, the older ammonia-soda plants continuing to operate very efficiently but no new ammonia-soda plants being built.