APRIL. 1947
IONS, SUBSTANCES, AND SUBSTANTIAL EQUATIONS ANTHONY STANDEN The Interscience Encyclopedia, Brooklyn, New York
SOME of the difficulty felt by many students in beginning chemistry may be due to their inarticulate attemuts to distineuish between facts and the theories whihh are supposed to explain the facts. Theri are students who "can't seem to get the hang of chemistry" although they may do well in other scientific subjects, such as biology. Such a student's uncertainty as to "what chemistry is about" may be due to his teacher's being uncertain as to what, quite literally, chemistry is about. Chemistry is the study of substances. If a teacher speaks of, say, "silver," it is quite clear what substance he is referring to. But if he speaks of "Ag+," then it is not clear, for he is in fact not referring to a substance, but to a hypothetical entity. He bannot show the students any Ag+. He can show them a a m b e r of substances in the stockroom which, he alleges, "contain Ag+." If he asserts, "All these suhstmces contain silver," this statement could fairly easily he proved by exhibiting the silver; hut the statement, "Allthese substances contain Ag+," cannot be so proved, for the Ag+ cannot be exhibited. Of course, chemistry students have to become accustomed fairly early in the course to dealing with hypothetical entities. If they read such an equation as it speaks of "HZ", which is a hypothetical entity, whose existence is only revealed to us by circumetantial evidence which is, of necessity, extremely indirect. equation such as
bridge to reality, are happier with equation (I), for they feel instinctively that this equation can be understood in a certain minimal sense as hydrogen plus oxygen gives water
a statement which is independent of a n y theory whatsoever and refers exclusively to facts. Equation (2) cannot be read in this minimal sense; it refers to facts as interpreted by particular theories. If the distinotion between'fact and theory he kept in mind, it will he seen that ions have a different status from substances. I n the series [Pt(NHa)a]++, [Pt(NH&Cl]+, Pt(NHs)zCls, [PtNHaClsl-, [PtClrl--, the five groups of symbols are alike in that they refer to molecular species, and the third,. which happens to be uncharged, has, as a molecular species, a status no different from the others. But the third is the only one which refers to a substance which can be isolated. We have reason to believe that the other four could be isolated if we could maintain matter a t a fantastically high intensity of electric charge; in other words, the "referent" (to use the semanticists' term) of the other four formulas is only hypothetically a substance. This throws light on a difficulty which is felt not only in teaching but also outside the classroom-namely, what to call an equation of the type SnCL
+ 2FeCL =SnC1, + 2FeCL
(3)
as opposed to the type of (2). Equation (2) is an ionic equation, but (3) is not well called a "molecular" equation, because the substmces referred to are not in all cases molecular species. Let equations of this type he Sn++ 2Fe+++=Sni' + 2Fef+ (2) called "substantial equations," for this is exactly what also deals with hv~otheticalentities whose status is in thev are: thev " sav. " ,as ionic eauations do not. exactlv no way inferior id that of "Hzv or 1i021'. But some whit substances are to he put'iuto the pot, A d whai students with tough minds, who like to maintain a firm substances will result if they react.
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