Page:The New International Encyclopædia 1st ed. v. 04.djvu/234

* CARBON COMPOUNDS. 194 CARBON COMPOUNDS. Bomewliat complex, and for this reason 'organic' clieniistry is kept separated from and taught after 'inorganic" chemistry. lsoL.Tiox OF CoMroi'NUS. The presence of one or more carbon compounds in a given substance may usually be delected by taking advantage of the fact that, on being heated, carbon compounds are charred, i.e. decomposed with separation of free carbon, which is readily recognized by its black color. The ne.t step in the examination of a given substance is to determine whether it consists of one single compuiind. or of a mix- ture of two or more compounds. This is usually accomplished by subjecting the substance to some physical process, such as distillation or crystal- lization, by means of which the substance may be separated into two or more portions; if these portions are found to be identical in their physi- cal and chemical properties, the conclusion is drawn that the substance consists of a single compound. (See CiiEMiSTKV.) In most cases distillation alone is capable of etTccting the com- plete separation of compounds, and the same may be said of the crystallization of solid substances from their solutions — another process frequently employed by chemists for isolating and purifying compounds of carbon. Crystallization is, as a rule, more effective as a process of purification than distillation, and hence chemists are anxious, whenever possible, to obtain their compounds in a solid crystalline form. A..^^LYSi.s. After a compound has been iso- lated and purified by repeated distillation or crystallization, its physical properties (especial- ly" the boiling or melting point) are carefully determined, and then it is analyzed. The analy- sis of carbon compounds usually involves the determination of carbon and hydrogen, and often of nitrogen. Carbon and hydrogen are deter- mined simultaneously by heating a known amount of the compound in a glass tube with copper oxide, which oxidizes all the hydrogen into ater vapor and all the carbon into carbonic acid. The process is for evident reasons t<-rmed a coih- husliun. The products of the combustion are col- lected, respectively, in sulphuric acid and in caustic potash, and their weights are carefully de- termined, the percentages of hydrogen and of car- bon in the given compound being calculated from those weights. A second combustion is required to determine the anunmt of nitrogen that may be present in the compound. The combustion yields the nitrogen in the free state, the gas being col- lected in a graduated tiibe which shows the vol- ume produced, and from this the percentage, by weight, of nitrogen in the given compound is found by a simple calculation. A direct deter- mination of oxygen is unnecessary; after the per- centages of carbon, hydrogen, and nitrogen have been determined, that of oxygen becomes evident. In the case of organic compounds of silver the amount of the latter may often be determined by simply heating a known quantity of the com- ])ound in a crucible ; the metal then remains bc- hindassuch, and is weighed directly, while the rest of the com]ioimd burns away. The amount of sodium or potassiiim in an organic compound may be determined by heating a known quantity of the latter with strong sulphuric acid, pure sodium or potassium sulphate being thus pro- duced, and fnmi the weight of this the percentage of .metal in the compound is readily calculated. The halogens (chlorine, bromine, and iodine) are best determined in organic compounds by heating these in sealed glass tubes with fuming nitric acid and silver nitrate, the product being a halo- gen compound of silver, from the weight of which the percentage of hydrogen in the given organic compound is found by a simple calculation. The determination of sulphur, phosphorus, and other elements that may sometimes be present in car- bon comiioiuuls need not be described here, the purpose of the present sketch being merely to convey a general idea of the ways in which il is possible to ascertain the composition of organic substances, and not to give detailed specific in- formation; such information should be sought in special works on chemical analysis. JIoLKCiLAB Formulas. After analysis has shown the percentage composition of a substance, the next stej) is to determine its molecular formula. For this purpose the percentages of the constituent elements must, first of all, be ex- pressed in terms of their atomic weights. Let, for example, an analysis of pure acetic acid give the following results: carbon, 39.9 per cent.; hydrogen, ti.7 i)er cent. ; and oxygen, 53.4 per cent. Since the atomic weights of the three elements are, respectively, 12, 1, and 10, the analvtical results 39.9 -i- 6.7 -r- 53.4 are writ- ten 'in the form (3.33 X 12) H- (ti.7 X 1) ^ (3.34X10), or, using the symbols of the ele- ments to denote their atomic weights, in the form, 3.33C -=- 6.7H -f- 3.340. Allowing for the errors of analysis, it is therefore evident tluit for every atom of carbon, acetic acid contains 2 atoms of hydrogen and 1 atom of o.xygen — a relation expressed by any one of the fornuilas, CH^O, C,H,0., CaHoOj, etc. According to the first of these formulas, the molecular weight of acetic acid would be 30 (i.e. 12Xl-flX2-flOXl); according to the second it would be 00 (i.e. 12 X 2+ 1 X 4-f 16 X 2) ; according to the third it would be 90, etc. Now, according to Avo- gadro's rule, the molecular weight of a com- pound is twice as great as the density of its vapor (compared with hydrogen). Therefore, in crder to fix the molecular weight of acetic acid, its vapor-density must be determined; and this may be done by one of the methods described in the' article Molecules — iIoLECUL.R Weights. The vajior-density being found to be about 30. the molecular weight is taken to be 60, and hence the formula C:H,02 is accepted as representing iv molecule of acetic acid. Chemical Constitution. The molecular formula of a compound represents its composition and its smallest relative reacting-weight. It is not, however, altogether characteristic of the compound, for nunu'rous cases are known in which a number of different compo ids are rei)re- sented by one and the same molecular fonnula. Thus, both ordinary alcohol and di-methyl ether (a substance that nu>y be obtained from wood- alcohol) are represented by the formula C.HoO; five compounds, viz. ordinary ether and the four different substances called butyl-alcohols, are found to have in common the formula C,H,c,0, etc. Such compounds are said to be isomeric, or more strictly, melunieric. the term isomeric being often extended to include also the so-called poh/merie compounds, i.e. those which have the same relative composition but not the same molecular weight, such as acetylene. C.Hj. and benzene, CoHj. (Another kind of isomerism may