Operations with Pure Chemical Substances

Almost all of the classification s main characteristics discussed in the previous chapters were given with or rested on its confinement to pure chemical substances. This confinement must be seen as a confinement to a particular domain of theory and practice, one which constituted the historical context of the Methode. As a manifold domain, which, as we will see, comprised different fields of chemical practice as well as theory, a dense description would be required for any portrayal to do justice to its complexity. Such a portrayal is, of course, beyond the scope of this chapter. However, an outline of this domain is indispensable, as its very existence as a distinguishable domain might legitimately be questioned due to its complexity. 

The different clusters of classes that we were able to distinguish in the table s classification are indicative of the chemical domain s extension in space and time, that is, its main fields of inquiry and its historical depth. We distinguished three such clusters, one around the conception of neutral salts including that of acids, another around the conception of gases, and a third around the conception of metal oxides and alloys. 

The most prominent figure in the field of pneumatic chemistry in the 1780s besides Lavoisier was Joseph Priestley, a well-known advocate of the phlogistic chemical system. 

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It is therefore obvious that the chemistry of pure substances can be defined only on the basis of its objects of inquiry. But it should be noted that in the eighteenth century it were the chemists themselves who distinguished these objects of inquiry from other ones in their practices of classification. We argue that their distinction corresponds exactly with the boundaries of objects of inquiry in the tableau of the Meth-ode. The question raised above was whether the authors of the Methode were the first to see an inner bond among the many different activities with pure chemical substances scattered through all domains of chemistry, or whether their distinction of the particular sphere of the chemistry of pure substances followed a tradition established earlier. Fortunately, there exists unmistakable evidence for such a tradition. The famous tables of chemical affinities testify unambiguously to the existence of this particular chemical practice. The first of these tables was the Table des differents rapports constmctedby Etienne Francois Geoffroy (1672-1731), published in 1718. We can thus even determine when the distinction of operations with pure chemical substances first became manifest, namely approximately seventy years before the Tableau of 1787. 

The molecular weights calculated in this way agree very closely with the so-called chemical molecular weights, derived from chemical analysis, and the method therefore rivals the latter in accuracy, without the attendant complications attaching to chemical operations with pure substances. 

If fossil fuels were composed of 100% pure carbon and hydrogen and all combustion processes operated with 100% chemical efficiency there would be no smog. The only combustion products would be water and carbon dioxide. These two substances are natural components of the atmosphere. They are non-toxic and transparent to visible light. Smog is produced by the reaction of the products of incomplete combustion with other natural and man made substances in the atmosphere. 

Chapters 8 and 9 are dedicated to the context and historical background of the Tableau. Chapter 8 explores the fields of chemical theory and practice in which the chemistry of pure chemical substances originated in the early modern period. We will particularly study operations with such substances in sixteenth- and seventeenth-century metallurgy and pharmaceutical salt production, as well as the interpretations of these operations by the historical actors, in which elements of the modern concept of chemical compound gradually developed. Around 1700, the bulk of these as yet independent and little-connected chemical operations using pure substances was integrated into a consistent system on the basis of the concept of replacement reaction. This integration, which established the chemistry of pure chemical substances as a special domain of chemical theory and practice, is affirmed by the affinity tables of... 

NFPA developed Standard 704 as a tool for identification and evaluation of potential hazards during emergency response, not for application to chemical process safety. The instability rating is a part of this standard. It was not intended to be used to measure reactivity, but rather to measure the inherent instability of a pure substance or product under conditions expected for product storage. The instability rating does not measure the tendency of a substance or compound to react with other substances or any other process-specific factors, such as operating temperature, pressure, quantity handled, chemical concentration, impurities with catalytic effects, and compatibility with other chemicals onsite. 

Preceding chapters have dealt largely with pure substances or with constant-composition mixtures. e.g., air. However, composition changes are the desired outcome, not only of chemical reactions, but of a number of industrially important mass-transfer operations. Thus composition becomes a primary variable in the remaining chapters of tliis text. Processes such as distillation, absorption, and extraction bring phases of different composition into contact, and when tlie phases are not in equilibriimi, mass transfer between the phases alters their compositions. Botli tlie extent of change and tlie rate of transfer depend on the departure of the system from equilibrium. Thus, for quantitative treatment of mass transfer the equilibrium T, P, and phase compositions must be known. 

Chemical Action. — Psychic influence is not, however, the sole cause of the secretion. When nutritive material is introduced into the stomach of an animal by the aid of a fistula, it is found that this food is more or less digested, indicating that the secretion is made even in the absence of all psychical intervention. The secretion produced in this last case is due to the specific action of the nutritive substances, and it is observed that the quantity and quality of the juice secreted vary with the chemical nature of the substance absorbed. Indeed, two causes regulate the secretion of the gastric juice, one of purely psychic origin, the other of chemical origin. To study the unconscious secretion, that is to say the direct effect produced by the foods on the secretorial glands, Pawlow introduced 400 g. of meat into the stomach of a dog operated upon as before. 

In summary, in the theoretical part of his Traicte Le Febvre divided the realm of materials first by grouping them into the two large classes of natural mixts and pure substances. He further classified the natural mixts according to their origin from the three natural kingdoms. His classification of pure substances—certain kinds of chemical preparations—relied on a taxonomic criterion that combined perceptible features of chemical operations with their interpretation as either purification or exaltation —a mode of classification also chosen by Libavius. The further subdivision of essences and other kinds of pure substances relied on their perceptible properties and medical virtues. 

One can also frequently choose between a purely mass-transfer operation and a chemical reaction or a combination of both. Water can be removed from an ethanol-water solution either by causing it to react with unslaked lime or by special methods of distillation, for example. Hydrogen sulfide can be separated from other gases either by absorption in a liquid solvent with or without simultaneous chemical reaction or by chemical reaction with ferric oxide. Chemical methods ordinarily destroy the substance removed, while mass-transfer methods usually permit its eventual recoveiy in unaltered form without great difficulty. 

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