CHEMISTRY 



element, different terminations and, where neces- 

 sary, other devices of nomenclature are employed 

 to distinguish amongst these. Thus there are two 

 acid anhydrides derived from sulphur sulphurous 

 anhydride, SO 2 , and sulphuric anhydride, SO 3 . The 

 latter unites with water to form sulphuric acid, 

 HjjSO,, and it is believed by some chemists that 

 the solution in water of sulphurous anhydride (a 

 gaseous substance) contains at least some of the 

 corresponding sulphurous acid, H 2 SO 3 . From sul- 

 phuric acid there is derived the series of ' salts 

 called sulphates, from sulphurous acid the series 

 called sulphites. It sometimes happens that an 

 acid and series of salts are known of which the 

 corresponding anhydride is unknown, just as the 

 existence of certain acids is doubtful although the 

 coi-responding anhydride is known. In other cases 

 series of salts are known, although both the corre- 

 sponding anhydride and acid are unknown. Certain 

 of these peculiarities, as well as some further forms 

 of nomenclature, are illustrated by the table given 

 below of the compounds corresponding to known or 

 unknown oxides of chlorine : 



Oxide. Acid. Salt. Name of salt. 



C1 2 O HC1O KC1O Potassium Hypochlorite. 

 HC1O 2 KC1O. 2 Chlorite. 



[C1O 2 , not an acid anhydride]. 

 HC1O 3 KC1O 3 Chlorate. 



HC1O 4 KC1O 4 Perchlorate. 



It has recently been proved that the substance 

 described in most text-books as chlorous anhydride, 

 C1 2 O 3 , is really a mixture, and that as yet C1 2 O 3 

 has not been prepared. The hypothetical chloric 

 and perchloric anhydrides would have the composi- 

 tion C1 2 O 5 and C1 2 6 7 respectively. 



A very large number of salts and other chemical 

 compounds are commonly known by popular names, 

 the latter being frequently of extremely ancient 

 origin. The popular name as a rule conveys no 

 information as to the composition of the substance. 

 For instance, copperas (ferrous sulphate, FeSOj 

 is not recognised oy its name as an iron compound, 

 nor calomel ( mercurous chloride, HgCl ) as a mer- 

 cury compound, nor litharge ( lead oxide, PbO ) as 

 a lead compound. It is the aim to convey, by the 

 systematic name of a substance, the greatest 

 possible amount of information as to its composi- 

 tion. It is not possible to attain to a perfect 

 system of nomenclature, as new discoveries render 

 changes necessary from time to time. 



Graphic Formulae. In addition to representing 

 the composition of a substance by means of 

 formulae, chemists endeavour to express certain 

 ideas as to the constitution, or arrangement of 

 the atoms in the molecule of substances by 

 means of graphic formulae. It must not be sup- 

 posed (as has sometimes erroneously been done) 

 that graphic formulas are intended to represent 

 the shape of molecules or the arrangement in 

 space of the atoms constituting such molecules, but 

 simply as a short method of expressing on paper 

 certain facts. No one supposes that a printed word 

 in any modern language is an attempt to draw the 

 object spoken of, or that it is more than a method 

 of representing on paper a given series of sounds, 

 and yet criticism based upon assumption scarcely 

 less absurd, has been directed against graphic for- 

 mulae. In a graphic formula we have the symbols 

 for the different elements grouped in a particular 

 way, so as ( 1 ) to indicate the valency ( see the 

 article ATOMIC THEORY) of each element, and 

 (2) to express ideas based upon observed facts 

 as to the most likely arrangement of the atoms 

 in a molecule, when various arrangements are 

 conceivable. 



The following may be given as simple illustra- 

 tions of ( 1 ) : 



H Cl, hydrochloric acid ; H -S H, sulphuretted 



/H 

 hydrogen ; Mg=O, magnesium oxide ; N^-H, am- 



H Cl 



monia ; H C H, marsh-gas ; H C Cl, chloro- 



H Cl 



PI 

 form; O=C<Cr<i> phosgene; O=C=O, carbonic 



anhydride ; S=C=S, carbon bisulphide, &c. The 

 letters representing monovalent atoms are written 

 with one stroke proceeding from them, those 

 representing divalent, trivalent, and tetravalent 

 atoms being written with two, three, and four such 

 strokes respectively. 

 Illustrations of (2) are : 



Urea. Ammonium cyanate. 



/N< H / H 



0=C< " and 0=C=N N^j 

 \N<g Xg 



These two substances illustrate two other points of 

 importance. One of these is the occurrence of the 

 nitrogen atom sometimes trivalent, as in ammonia, 

 NH 3 , sometimes pentavalent, as in the ammoniuia 

 salts e.g. ammonium chloride, NH 4 C1. In am- 

 monium cyanate one atom of nitrogen is represented 

 as trivalent and the other as pentavalent. The- 

 two substances, moreover, illustrate Isomerism 

 (q.v.), or the existence of two or more com- 

 pounds containing exactly the same elements and 

 in the same proportions, and yet differing from one 

 another in chemical and physical properties. 



Chemical Changes. There are several kinds 

 of chemical changes which are of very frequent 

 occurrence, and may conveniently be classified. 

 The simple union of one element with another 

 has already been mentioned, and closely related 

 to this kind of change is the union of a compound 

 with an element or with another compound. 

 Along with these changes may be classed those in 

 which a compound breaks up into two or more 

 elements or simpler compounds, or into one or 

 more of each. All these variations are illustrated 

 by the following equations : 



C +0 2 =C0 2 } 



CO + Cl, = COC1 2 Uirect union. 



CaO + C0. 2 = CaCOj 



2HgO = 2Hg + O 2 }, 



(NH 4 ),Cr. 2 7 = Cr 2 s + 4H 2 O + N 2 Uecomposition 

 Ca(HO) 2 = CaO + H 2 O 2 J( by heating). 



One of the most important kinds of chemical 

 change is that called double decomposition. This 

 occurs perhaps most frequently when solutions of 

 salts are mixed with each other, and it is charac- 

 terised by a mutual exchange of metal and salt 

 radical. If an aqueous solution of sodium chloride 

 be mixed with one of potassium bromide, although 

 no visible change takes place, we have reason to 

 believe that double decomposition goes on to a 

 certain extent, with formation of some sodium 

 bromide and some potassium chloride, whilst some 

 of each of the original salts also remains, a state 

 of equilibrium being eventually established amongst 

 the four salts. If, however, one of the new pro- 

 ducts formed by double decomposition be insoluble 

 or practically insoluble in water, as soon as any of 

 it is formed it will appear as a precipitate, and be 

 thus removed from solution, so that no condition of 

 equilibrium can be established until formation of a 

 precipitate no longer occurs i.e. until the double 

 decomposition is complete. Thus, if solutions of 

 sodium chloride and silver nitrate be mixed in the 

 proper proportions, the extremely insoluble silver 



