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CHEMISTRY (formerly "chymistry"; Gr. ...
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See also:CHEMISTRY (formerly "chymistry"; Gr. xvµela; for derivation see See also:ALCHEMY) , the natural See also:science which has for its See also:province the study of the See also:composition of substances . In See also:common with physics it includes the determination of properties or characters which serve to distinguish one substance from another, but while the physicist is concerned with properties possessed by all substances and with processes in which the molecules remain intact, the chemist is restricted to those processes in which the molecules undergo some See also:change . For example, the physicist determines the See also:density, See also:elasticity, hardness, See also:electrical and thermal conductivity, thermal expansion, &c.; the chemist, on the other See also:hand, investigates changes in composition, such as .See also:nay be effected by an electric current, by See also:heat, or when two or more substances are mixed . A further differentiation of the provinces of See also:chemistry and physics is shown by the classifications of See also:matter . To the physicist matter is presented in three leading forms—solids, liquids and gases; and although further sub-divisions have been rendered necessary with the growth of knowledge the same principle is retained, namely, a See also:classification based on properties having no relation to composition . The fundamental chemical classification of matter, on the other hand, recognizes two See also:groups of substances, namely, elements, which are substances not admitting of See also:analysis into other substances, and compounds, which do admit of analysis into simpler substances and also of See also:synthesis from simpler substances . Chemistry and physics, however, meet on common ground in a well-defined See also:branch of science, named See also:physical chemistry, which is primarily concerned with the correlation of physical properties and chemical composition, and, more generally, with the elucidation of natural phenomena on the molecular theory . It may be convenient here to See also:state how the whole subject of chemistry is treated in this edition of the See also:Encyclopaedia Britannica . The See also:present See also:article includes the following sections: I . See also:History.—T his See also:section is confined to tracing the See also:general trend of the science from its See also:infancy to the See also:foundations of the See also:modern theory . The history of the alchemical See also:period is treated in more detail in the article See also:ALCHEMY, and of the iatrochemical in the article See also:MEDICINE . The See also:evolution of the notion of elements is treated under See also:ELEMENT; the molecular See also:hypothesis of matter under See also:MOLECULE; and the See also:genesis of, and deductions from, the atomic theory of See also:Dalton receive detailed analysis in the article See also:ATOM . II . Principles.—This section treats of such subjects as nomenclature, formulae, chemical equations, chemical change and similar subjects . It is intended to provide an introduction, necessarily brief, to the terminology and machinery of the chemist . VI . 2 IV . Organic Chemistry.—This section includes a brief history of the subject, and proceeds to treat of the principles underlying the structure and interrelations of organic compounds . V . See also:Analytical Chemistry.—This section treats of the qualitative detection and separation of the metals, and the commoner methods employed in quantitative analysis . The analysis of organic tom-pounds is also noticed . VI . Physical Chemistry.—This section is restricted to an See also:account of the relations existing between physical properties and chemical composition . Other branches of this subject are treated in the articles CHEMICAL See also:ACTION; See also:ENERGETICS; See also:SOLUTION; Annoys; See also:THERMOCHEMISTRY . I . HISTORY Although chemical actions must have been observed by See also:man in the most remote times, and also utilized in such processes as the extraction of metals from their ores and in the arts of tanning and See also:dyeing, there is no See also:evidence to show that, beyond an unordered accuml}lation of facts, the See also:early developments of these See also:industries were attended by any real knowledge of the nature of the processes involved . All observations were the result of See also:accident or See also:chance, or possibly in some cases of experimental trial, but there is no See also:record of a theory or even a general classification of the phenomena involved, although there is no doubt that the ancients .had a See also:fair knowledge of the properties and uses of the commoner substances . The origin of chemistry is intimately See also:bound up with the arts which we have indicated; in this respect it is essentially an experimental science . A unifying principle of chemical and physical changes was provided by metaphysical conceptions of the structure of matter . We find the notion of " elements," or See also:primary qualities, which confer upon all See also:species of matter their distinctive qualities by appropriate See also:combination, and also the See also:doctrine that See also:Greek matter is composed of See also:minute discrete particles, See also:plum . prevailing in the Greek See also:schools . These " elements, sophy . sops however, had not the significance of the elements of to-See also:day; they connoted physical appearances or qualities rather than chemical relations; and the atomic theory of the ancients is a See also:speculation based upon metaphysical considerations, having, in its origin, nothing in common with the modern molecular theory, which was based upon experimentally observed properties of gases (see ELEMENT; MOLECULE) . Although such hypotheses could contribute nothing directly to the development of a science which laid especial claim to experimental investigations, yet indirectly they stimulated inquiry into the nature of the " essence " with which the four " elements " were associated . This quinta essentia had been speculated upon by the Greeks, some regarding it as immaterial or aethereal, and others as material; and a school of philosophers termed alchemists arose who attempted the See also:isolation of this essence . The existence of a fundamental principle, unalterable and indestructible, prevailing alike through physical and chemical changes, was generally accepted .
Any change which a substance may chance to undergo was simply due to the discarding or taking up of some proportion of the primary " elements " or qualities: of these coverings " See also:water," " See also:air," " See also:earth " and " See also:fire " were regarded as clinging most tenaciously to the essence, while " See also:cold," " heat," " moistness " and " dryness " were more easily See also:cast aside or assumed
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Several origins have been suggested for the word alchemy, and there seems to Alchemy. have been some doubt as to the exact nature and
import of the alchemical doctrines
.
According to M
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P
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E
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See also:Berthelot, " alchemy rested partly on the See also:industrial processes of the See also:ancient Egyptians, partly on the speculative theories of the Greek philosophers, and partly on the mystical reveries of the Gnostics and Alexandrians." The See also:search for this essence subsequently resolved itself into the See also:desire to effect the trans-mutation of metals, more especially the See also:base metals, into See also:silver and See also:gold
.
It seems that this secondary principle became the dominant See also:idea in alchemy, and in this sense the word is used in See also:Byzantine literature of the 4th See also:century; Suidas, See also:writing in
II
the 1 rth century, defines chemistry as the " preparation of silver and gold " (see ALCHEMY)
.
From the Alexandrians the science passed to the See also:Arabs, who made discoveries and improved various methods of separating substances, and afterwards, from the 11th century, became seated in See also:Europe, where the alchemical doctrines were assiduously studied until the 15th and 16th centuries
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It is readily understood why men imbued with the authority of tradition should prosecute the search for a substance which would confer unlimited See also:wealth upon the fortunate discoverer
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Some alchemists honestly laboured to effect the transmutation and to discover the " philosopher's See also:
It is really not extraordinary that See also:Isaac Hollandus was able to indicate the method of the preparation of the " philosopher's stone " from " adamic " or " virgin " earth, and its action when medicinally employed; that in the writings assigned to See also:Roger See also:
At the same time he clarified the conception of elements and compounds, rejecting the older notions, the four elements of the " vulgar Peripateticks " and the three principles of the vulgar Stagyrists," and defining an element as a substance incapable of decomposition, and a See also:compound as composed of two or more elements
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He explained chemical combination on the hypotheses that matter consisted of minute corpuscles, that by the coalescence of corpuscles of different sub-stances distinctly new corpuscles of a compound were formed, and that each corpuscle had a certain See also:affinity for other corpuscles
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Although Boyle practised the methods which he expounded, he was unable to gain general See also:acceptance of his doctrine of
elements; and, strangely enough, the theory which 1Ytogl next dominated chemical thought was an alchemical Ptheorystic
invention, and lacked the lucidity and perspicuity
of Boyle's views
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This theory, named the phlogistic theory, was primarily based upon certain experiments on See also:combustion and calcination, and in effect reduced the number of the alchemical principles, while setting up a new one, a principle of combustibility, named phlogiston (from 4,lwyurros, burnt)
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Much discussion had centred about fire or the "igneous principle." On the one hand, it had been held that when a substance was burned or calcined, it combined with an " air "; on the other hand, the operation was supposed to be attended by the destruction or loss of the igneous principle
.
Georg See also:Ernst See also:Stahl, following in some measure the views held by Johann See also:Joachim See also:Becher, as, for instance, that all combustibles contain a " sulphur " (which notion is itself of older date than Becher's terra pinguis), regarded all substances as capable of See also:resolution into two components, the inflammable principle phlogiston, and another element—" water," " See also:acid " or " earth." The violence or completeness of combustion was proportional to the amount of phlogiston present
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Combustion meant the liberation of phlogiston
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Metals on calcination gave calces from which the metals could be recovered by adding phlogiston, and experiment showed that this could generally be effected by the action of See also:coal or See also:carbon, which was therefore regarded as practically pure phlogiston; the other constituent being regarded as an acid
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At the hands of Stahl and his school, the phlogistic theory, by exhibiting a fundamental similarity between all processes of combustion and by its remarkable flexibility, came to be a general theory of chemical action
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The objections of the antiphlogistonists, such as the fact that calces weigh more than the See also:original metals instead of less as the theory suggests, were answered by postulating that phlogiston was a principle of levity, or even completely ignored as an accident, the change of qualities being regarded as the only matter of importance
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It is remarkable that this theory shouldhave gained the esteem of the notable chemists who flourished in the 18th century
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See also: See also:Black, K . W . See also:Scheele, A . S . See also:Marggraf, J . See also:Priestley and many others who might be mentioned . Libavius (d . 1616), chiefly famous for his See also:Opera Omnia Medicochymica (1595) ; See also:Jean See also:Baptiste See also:van See also:Helmont (1577-1644), celebrated for his researches on gases ; F. de la Boe Sylvlus (1614-1672), who regarded' medicine as applied chemistry; and See also:Otto Tachenius, who elucidated the nature of salts . Descriptive chemistry was now assuming considerable See also:pro-portions; the experimental inquiries suggested by Boyle were See also:Lavoisier. being assiduously See also:developed; and a wealth of observa- tions was being accumulated, for the explanation of which the resources of the dominant theory were sorely taxed . To quote See also:Antoine See also:Laurent Lavoisier, " . . . chemists have turned phlogiston into a vague principle, . . . which consequently adapts itself to all the explanations for which it may be required .
Sometimes this principle has See also:weight, and sometimes it has not; sometimes it is See also:free fire and sometimes it is fire combined with the earthy element; sometimes it passes through the pores of vessels, sometimes these are impervious to it; it explains both causticity and non-causticity, transparency and opacity, See also:colours and their See also:absence; it is a veritable See also:Proteus changing in See also:form at each instant." Lavoisier may be justly regarded as the founder of modern or quantitative chemistry
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First and foremost, he demanded that the See also:balance must be used in all investigations into chemical changes
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He established as fundamental that combustion and calcination were attended by an increase of weight, and concluded, as .did Jean Rey and See also: The same results were obtained with lead and tin; and a more elaborate repetition indubitably established their correctness . He also showed that on See also:heating mercury calx alone an " air " was liberated which differed from other " airs," and was slightly heavier than ordinary air; moreover, the weight of the " air " set free from a given weight of the calx was equal to the weight taken up in forming the calx from mercury, and if the calx be heated with See also:charcoal, the metal was recovered and a See also:gas named " fixed air," the modern carbon dioxide, was formed . The former experiment had been performed by Scheele and Priestley, who had named the gas "phlogisticated air "; Lavoisier subsequently named it oxygen, regarding it as the " acid producer " (oEus, sour) . The theory advocated by Lavoisier came to displace the phlogistic conception; but at first its acceptance was slow . Chemical literature was full of the phlogistic modes of expression—oxygen was " dephlogisticated air," nitrogen " phlogisticated air," &c.—and this tended to retard its promotion . Yet really the transition from the one theory to the other was See also:simple, it being only necessary to change the " addition or loss of phlogiston " into the " loss or addition of oxygen." By his insistence upon the use of the balance as a quantitative check upon the masses involved in all chemical reactions, Lavoisier was enabled to establish by his own investigations and the results achieved by others the principle now known as the " conservation of See also:mass." Matter can neither be created nor destroyed; however a chemical See also:system be changed, the weights before and after areequal.l To him is also due a rigorous examination of the nature of elements and compounds; he held the same views that were laid down by Boyle, and with the same prophetic foresight predicted that some of the elements which he himself accepted might be eventually found to be compounds . It is unnecessary in this See also:place to recapitulate the many results which had accumulated by the end of the 18th century, or to discuss the labours and theories of individual workers since these receive attention under See also:biographical headings; in this article only the salient features in the history of our science can be treated . The beginning of the 19th century was attended by far-reaching discoveries in the nature of the composition of compounds . Investigations proceeded in two directions:—(1) the nature of chemical affinity, (2) the See also:laws of chemical combination . The first question has not yet been solved, although it has been speculated upon cnem/ca/ a//m/ty. from the earliest times . The alchemists explained chemical action by means of such phrases as " like attracts like," substances being said to combine when one " loved " the other, and the See also:reverse when it " hated " it . Boyle rejected this terminology, which was only strictly applicable to intelligent beings; and he used the word " affinity" as had been previously done by Stahl and others .
The modern sense of the word, viz. the force which holds chemically dissimilar substances together (and also similar substances as is seen in di-, tri-, and poly-atomic molecules), was introduced by See also:Hermann See also:Boerhaave, and made more precise by See also:Sir Isaac See also:Newton
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The laws of chemical combination were solved, in a measure, by John Dalton, and the solution expressed as Dalton's " atomic theory." Lavoisier appears to have assumed that the composition of every chemical compound was See also:constant, and the same opinion was the basis of much experimental inquiry at the hands of See also:Joseph See also: a The theory of Berthollet was essentially See also:mechanical, and he attempted to prove that the course of a reaction depended not on See also:affinities alone but also on the masses of the reacting components . In this respect his hypothesis has much in common with the " law of mass-action " developed at a much later date by the See also:Swedish chemists Guldberg and Waage, and the See also:American, See also:Willard See also:Gibbs (see CHEMICAL ACTION) . In his classical thesis Berthollet vigorously attacked the results deduced by See also:Bergman, who had followed in his table of elective attractions the path traversed by Stahl and S . F . See also:Geoffroy . elements received symbols composed of circles, arcs of circles, and lines, while certain class symbols, such as 'tZ' for metals, +f or acids, for alkalies, c for salts,/ for calces, &c., were used . Compounds were represented by copulating simpler symbols, e.g. mercury calx was .3 Bergman's symbolism was obviously cumbrous, and the system used in 1782 by Lavoisier was equally abstruse, since the forms gave no See also:clue as to composition; for instance water, oxygen, and nitric acid were 7 +i, and es .. deduced the relative weight of the oxygen atom to be 6.5; from See also:marsh gas and olefiant gas he deduced carbon = 5, there being one atom of carbon and two of hydrogen in the former and one atom of hydrogen to one of carbon in the latter; nitrogen had an equivalent of 5, and so on.' The value of Dalton's generalizations can hardly be over-estimated, notwithstanding the fact that in several cases they needed correction . The first step in this direction was effected by the co-ordination of See also:Gay Lussac's observations on the combining volumes of gases . He discovered that gases always combined in volumes having simple ratios, and that the See also:volume of the product had a simple ratio to the volumes of the reacting gases . For example, one volume of oxygen combined with two of hydrogen to form two volumes of See also:steam, three volumes of hydrogen combined with one of nitrogen to give two volumes of See also:ammonia, one volume of hydrogen combined with one of See also:chlorine to give two volumes of hydrochloric acid . An immediate inference was that the Daltonian " atom " must have parts which enter into combination with parts of other atoms; in other words, there must exist two orders of particles, viz .
(i) particles derived by limiting mechanical subdivision, the modern molecule, and (2) particles derived from the first class by chemical subdivision, i.e. particles which are incapable of existing alone, but may exist in combination
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Additional evidence as to the structure of the molecule was discussed by See also:Avogadro in 1811, and by See also:Ampere in 1814
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From the gas-laws of Boyle and J
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A
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C
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See also:
This may be due in some measure to the small number of gaseous and easily volatile substances then known, to the attention which the study of the organic compounds received, and especially to the energetic investigations of J
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J
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Berzelius, who, fired with See also:enthusiasm by the original theory of Dalton and the law of multiple proportions, determined the equivalents of combining ratios of many elements in an enormous number of compounds.2 He prosecuted his labours in this See also: The " compound acidifiable bases," i.e. the hypothetical radicals of acids, were denoted by squares enclosing the initial See also:letter of the base; an See also:alkali was denoted by a triangle, and the particular alkali by inserting the initial letter . Compounds were denoted by joining the symbols of the components, and by varying the manner of joining compounds of the same elements were distinguished . The symbol V was used to denote a liquid, and a See also:vertical See also:line to denote a gas . As an example of the complexity of this system we may note the five oxides of nitrogen, which were symbolized as the first three representing the gaseous oxides, and the last two the liquid oxides . A great advance was made by Dalton, who, besides introducing simpler symbols, regarded the symbol as representing not only the element or compound but also one atom of that element or compound; in other words, his symbol denoted equivalent weights.' This system, which permitted the correct representation of molecular composition, was adopted by Berzelius in 1814, who, having replaced the geometric signs of Dalton by the initial letter (or letters) of the Latin names of the elements, represented a compound by placing a plus sign between the symbols of its components, and the number of atoms of each component (except in the See also:case of only one atom) by placing Arabic numerals before the symbols; for example, copper See also:oxide was Cu +0, sulphur trioxide S+30 . If two compounds combined, the + signs of the free compounds were discarded, and the number of atoms denoted by an Arabic See also:index placed after the elements, and from these modified symbols the symbol of the new compound was derived in the same manner as simple compounds were built up from their elements . Thus copper sulphate was CuO+SO3, See also:potassium sulphate 2S03+PoO2 (the symbol Po for potassium was subsequently discarded in favour of K from kalium) . At a later date Berzelius denoted an oxide by dots, equal in number to the number of oxygen atoms present, placed over the element; this notation survived longest in See also:mineralogy . He also introduced barred symbols, i.e. letters traversed by a See also:horizontal See also:bar, to denote the See also:double atom (or molecule) . Although the system of Berzelius has been modified and extended, its principles survive in the modern notation . In the development of the atomic theory and the deduction of the atomic weights of elements and the formulae of compounds, Dalton's arbitrary rules failed to find See also:complete accept- See also:Extension ante . Berzelius objected to the hypothesis that if of the two elements form only one compound, then the at"' atoms combine one and one; and although he agreed theory . with the See also:adoption of simple rules as a first See also:attempt at representing a compound, he availed himself of other data in See also:order to gain further See also:information as to the structure of compounds . For example, at first he represented ferrous and ferric oxides by the formulae FeO,, FeO,, and by the analogy of See also:zinc and other basic oxides he regarded these substances as constituted similarly to FeO,, and the acidic oxides alumina and See also:chromium oxide as similar to FeO, . He found, however, that chromic acid, which he had represented as CrOs, neutralized a base containing s the 3 The following symbols were also used by Bergman: b, Yf, °--°, V, , which represented zinc, See also:manganese, See also: |