Substances, definitions

The definition of reference thermometric fixed point is an equilibrium state of a definite substance the realization of a fixed point must depend only on the composition and on the substance . Hence boiling points, for example, cannot be considered fixed points, since they depend on pressure. Only triple points fulfil this definition as can be deduced from the Gibbs rule for pure substances ... 

THE RECOGNITION OF CHEMICAL CHANGES AS THE INTERACTIONS OF DEFINITE SUBSTANCES. 

Lavoisier also recognised that certain definite substances could be divided into things simpler than themselves, but that other substances refused to undergo simplification by division into two or more unlike portions. He spoke of the object of chemistry as... 

When the student of chemistry desires to effect the transformation of one definite substance into another, he is told to determine, by quantitative experiments, what are the elements, and what the quantities of these elements, which compose the compound which he proposes to change, and the compound into which he proposes to change it and he is given working definitions of the words element and compound. If the compound he desires to produce is found to be composed of elements different from those which form the compound wherewith his operations begin, he is directed to bring about a reaction, or a series of reactions, between the compound which is to be changed, and some other collocation of elements the composition of which is known to be such that it can supply the new elements which are needed for the production of the new compound. 

A chemical element was thought of by Lavoisier as "the actual term whereat analysis has arrived," a definite substance "which we cannot subdivide with our present knowledge," but not necessarily a substance which will never be divided. A compound was thought of by him as a definite substance which is always produced by the union of the same quantities of the same elements, and can be separated into the same quantities of the same elements. 

Since chemists realised the meaning of Dalton s book, published in 1808, and entitled, A New System of Chemical Philosophy, elements have been regarded as distinct and definite substances, which have not been divided into parts different from themselves, and unite with each other in definite quantities by weight which can be accurately expressed as whole multiples of certain fixed quantities and compounds have been regarded as distinct and definite substances which are formed by the union of, and can be separated into, quantities of various elements which are expressible by certain fixed numbers or whole multiples thereof These descriptions of elements and compounds are expressions of actual facts. They enable chemists to state the compositions of all the compounds which are, or can be, formed by the union of any elements. For example, let A, B, C, and D represent four elements, and also certain definite weights of these elements, then the compositions of all the compounds which can be formed by the union of these elements are expressed by the scheme A Bm Cp Dg, where m n p and q are whole numbers. 

This sounds like a grand definition. It is, and one can see what it means by looking at Figs. 7.1 and 7.2. In Fig. 7.1, one sees the first part of the definition (substances from electricity). Copper ions, invisible and dissolved in solution, are converted into visible metallic copper by means of the electrons flowing across the interfaces to the copper ions in solution. A new substance is produced by means of the flow of electricity. In Fig. 7.2, the reverse occurs One puts in a substance at one electrode and another substance at the other, and gets electricity So, electrochemistry has (as its name suggests), a chemical and an electrical side. 

Modification II and V are so much alike that. F. Wallerant suggested that they are identical. He said II n y aurait rien de surprenant dans 1 existence de deux modifications appartenant au mSme systeme et parallelement orientees.. . . Mais bien pins il n y a en realite qu une modification stable dans deux intervalles de temperature. If this hypothesis were to be verified the phenomenon would be unique. No case is known where the crystals of a single definite substance, presenting the same crystalline form with all the physical properties identical, can exist within two separate ranges of temp. 

Tellurium oxychloride is probably non-existent as a definite substance, the compound which has been described probably having been a mixture of chloride and oxide.6... 

Today Vieille s formulae are regarded as purely conventional though useful in arranging various types of nitrocellulose into classes. Nevertheless, many authors have tried to isolate definite substances corresponding to Vieille s compounds. Thus de Bruin [6] claimed that he had isolated definite compounds containing 11.97 and 12.76% of nitrogen, corresponding to the enneanitrocellulose and deca-nitrocellulose of Vieille. 

Definite substances can be identified by their IR and Raman spectra, interpreted like fingerprints. 

In order to bring out the phenomena of equilibrium in their simplest form, let us follow the changes of which a definite substance is capable, which can only change physically, that is only in regard to its state of aggregation. We can obtain a picture of the phenomena wo are considering from the observations of Kamsay and... 

As the number of known chemical compounds neared the million mark, literature about them and less definite substances or products became increasingly difficult to index. Chemical Abstracts and Chemisches Zentralblatt responded with improvements in entry selection and arrangement, and explained their improvements to searchers (5, 7). The German science press of the early 1930 s warmly praised the Zentralblatt s new encyclopedic system, which departs further from the strict alphabetic base than does the Chemical Abstracts system. 

The tendency for a specific substance to lose or gain electrons is called its redox or reduction potential. The redox potential of a conjugate redox pair is measured in an electrochemical cell against a reference standard, usually a standard hydrogen electrode. The redox potential of the standard hydrogen electrode is 0.0 V at 1 atm, by definition. Substances with a more negative... 

Tannins are, by definition, substances capable of producing stable combinations with proteins and other plant polymers such as polysaccharides. The transformation of animal skins into rotproof leather results from this property, as does astringency, fining and enzyme inhibition. Tannins react with proteins in each instance collagen in tanning. 

A yield of yi per cent, of dibromopropionyI-C37stine was obtained by Fischer and Suzuki and was apparently a definite substance. It was therefore regarded as the dl-compound, since its formation is independent of the formation of the dd- or ll-compounds which most probably would result in equal amounts. 

In military operations, the optical behavior of smokes and more recently the dispersion and dissipation of definite substances as aerosols have become of great importance. If light is thrown on a smoke cloud whose particles are approximately of the size of its wavelengths, and if... 

Definition Substance of volcanic origin consisting chiefly of complex silicates of aluminum and alkali metals... 

Definition Substance obtained by removal of lactose from whey as fluid, cone., or dried Uses Food additive, binder Regulatory FDA 21CFR 135.110, 135.140,... 

Purification of the Rohferment gives preparations 10 to 16 times more active (jS-glucosidase value 10 to 16) and some of the enzyme constituents of the cruder preparations are lost in the purification process. It should be noted that almond emulsin is not a definite substance but is a mixture of enzymes which are present in variable proportions depending on the method and extent of purification. Various methods for the purification may be used... 

---