Chapter V: Compound Substances

In a previous chapter a list of elementary substances has been given. All other matters known are compounds. From what has been said already, it is evident that the compounds are very numerous, indeed that there is practically no limit to the number of possible ones. These compounds are all made up by the union of elementary substances in obedience to the peculiar chemical forces that reside within them. Some compounds have only two kinds of elements: they are called binaries. Some compounds have three kinds of elements: they are called ternaries. Other compounds may have four, five, six, or even more kinds of elements grouped together to form one sort of substance. In this place reference will be made principally to binaries and ternaries—that is to the compounds of the simpler forms of constitution.

Examples of Binary Compounds.

In discussing binaries it will be well to give at the outset three or four examples for the purpose of illustration.

_First._ The gas known as hydrogen and the gas known as chlorine have the power of combining chemically and producing an entirely new compound, a compound different from hydrogen and different from chlorine yet containing portions of each of them. This compound is a binary since it consists of but two kinds of elements. It has several names, one of which is _hydric chloride_. The chemist frequently represents what is evidently the smallest possible quantity of this substance, and also its exact composition, by the expression

H Cl.

It is plain that this expression means a minute portion of substance formed by the union of one atom of hydrogen, (expressed by H,) and one atom of chlorine, (expressed by Cl).

_Second._ When sulphur burns in the air, it produces a blue flame. At the same time a new and peculiar gas is formed which is easily recognized by its choking odor, similar to that given off by a burning sulphur match. Now this odor is one of the properties of a new compound that has been formed: a compound different from sulphur, different from oxygen, yet containing them both and produced by the union of them. The compound is a binary because it contains but two kinds of elements. It is called _sulphur dioxide_. The name is intended to suggest that there are two atoms of oxygen to one of sulphur in the compound. This idea is further conveyed by the abridged system of notation so commonly used by chemists. Thus by this system the smallest possible quantity of the compound in question is expressed as follows,

SO₂.

In this expression it is very plain that S stands for one atom of sulphur, and O₂ for two atoms of oxygen.

_Third._ But sulphur may be made to combine with a still larger amount of oxygen than it takes when it simply burns in the air. Then it forms a compound called _sulphur trioxide_. This is still a binary, since it contains nothing but sulphur and oxygen, that is only two elementary substances. Expressed in the briefer form the smallest quantity of this compound may be represented by the formula,

SO₃.

This expression means a compound arising from the union of one atom of sulphur and three atoms of oxygen.

_Fourth._ When lead is heated to the melting point it is observed to become coated with a constantly increasing mass of a kind of ashes. A pound of the lead when heated in this way produces considerably more than a pound of dross. The formation of this dross is explained by the fact that when lead is heated it really burns, though of course the rapidity of the burning depends upon the amounts of heat and air to which the lead is subjected. Evidently the lead, in burning, has something added to itself. That something is a gas which is ever present in the atmosphere and which is called oxygen. The dross is a chemical compound of lead and oxygen. It is called _plumbic oxide_, and its smallest quantity is represented by the formula,

PbO.

In this formula it is easy to see that Pb stands for an atom of lead (whose Latin name is _plumbum_), and O for an atom of oxygen. The dross then is a binary compound.

A multitude of such examples of binary compounds might be given; probably those already cited are sufficient for the present. It will be advantageous to the reader to carefully learn the names and the formulas of the binary compounds thus far given, since they are selected examples which may be used again further on.

Examples of Ternary Compounds.

The ternary compounds are those which consist of three kinds of elements; of course they are more complicated in structure than the binaries. This fact, however, must not deter the reader from the attempt to understand them at the outset, for the principal ternaries are acids and salts, and everyone knows that acids and salts are among the most important compounds which the chemist has to employ.

As examples of ternary acids mention will be made of two of the principal ones used by the chemist.

And first, _nitric acid_ is a compound of hydrogen, nitrogen and oxygen. The formula of the smallest individual portion of it is

HNO₃.

These letters signify that nitric acid contains one atom of hydrogen, combined with one atom of nitrogen and three atoms of oxygen. Now this nitric acid forms a great many salts. A simple example may be found in that one containing silver. Thus when nitric acid and silver are warmed together, either a part or the whole of the silver dissolves. A new substance is produced which is commonly called nitrate of silver. By the chemist it is oftener called argentic nitrate. Its solution may be dried into the form of a white crystalline substance, one that has long been accepted as a member of the class of salts. Its formula is

AgNO₃.

It is plain that this last formula is employed as a short way of expressing that the salt is a compound of more than one kind of element—of three kinds in fact—and that these elements are in the proportions of one atom of silver, one atom of nitrogen, and three atoms of oxygen.

Promise was made to refer to two important acids; _sulphuric acid_ is the second one. Commercially this substance is by far the most important of all the acids. Indeed its manufacture is one branch of the greatest chemical industry devised by man—the alkali trade. Evidently it is important that the chemist should be thoroughly acquainted with sulphuric acid, with its composition, its formula, its way of chemically acting on other substances, and the things or products that it gives rise to when it has opportunity so to act. Now sulphuric acid has the formula

H₂SO₄.

This formula means that sulphuric acid is a ternary, being made up of three different kinds of elements, namely two atoms of hydrogen, one atom of sulphur and four atoms of oxygen.

Further sulphuric acid forms a multitude of salts. Thus it forms one containing silver. This is commonly called sulphate of silver, though the chemist generally calls it argentic sulphate. The formula of argentic sulphate is

Ag₂SO₄

When this formula is firmly acquired by the reader so that he can readily compare it with others already mentioned, a certain simple and distinct relationship may be traced. Thus comparing

Argentic sulphate, Ag₂SO₄
with Sulphuric acid, H₂SO₄

it is evident that in the one, two atoms of silver have taken the places of two atoms of hydrogen that appeared in the other. And such is usually the case: when silver takes the place of hydrogen, it does so, atom for atom. Indeed argentic nitrate, AgNO₃, already described, illustrates this fact. It is a compound product closely related to nitric acid, HNO₃, the only difference of construction being that here also one atom of silver has taken the place of one atom of hydrogen.

The Purpose of this Chapter.

The purpose of this chapter has been to suggest a few facts respecting the nature of chemical compounds and also to show how the science of chemistry employs its peculiar language both in its longer and shorter forms. This language is very comprehensive. In fact it is too elaborate for full explanation here. The plan contemplated is to give at this point a few hints as to its nature and scope, and to develop it only so far as may be necessary to the succeeding stages of our progress.

It is proper to suggest at this point that no single scientific man—nor society of them—can _enforce_ the use of any particular words upon the great body of chemists. For this reason, as well as for others, there still prevails the use of different chemical names for the same substance. Thus the compound of hydrogen and chlorine first referred to as represented by the formula HCl, has at least four widely used names: _first_, a name merely suggestive of its component parts, that is hydric chloride; _second_, names which suggests something in addition to its component parts, namely that it is an acid, thus it is called both chlorohydric acid and hydrochloric acid; _third_, an old fashioned name, which still retains its hold upon the commercial world, namely, muriatic acid.

This same general principle applies to a great many other substances, and while it is true that it thus increases the number of names in the chemical language, it likewise incidentally enriches that language. For in many cases it has come to pass, little by little, that these different names are appropriated to slightly differing forms of the same substance, and so the name employed often conveys to the intelligent chemist as definite a shade of meaning as do the different synonyms used in the descriptions of every-day affairs by any accomplished author. A single example will elucidate this point. The term oil of vitriol would usually be defined as meaning sulphuric acid. But the words sulphuric acid convey, strictly speaking, the same meaning as the formula

H₂SO₄

This latter substance, however, is of very rare occurrence alone: it is usually associated with varying quantities of water, and is then spoken of as sulphuric acid of varying degrees of dilution. Now in commerce the term oil of vitriol has come to be appropriated exclusively to that dilution consisting of about

89 per cent. of sulphuric acid, H₂SO₄
with 11 per cent. of water, H₂O,

both taken by weight.

READING REFERENCES.

Notation, Chemical

=Frankland=, E.—Experimental Researches in Chemistry. 3.
London. 1877.
=Williamson=, A. W.—Jour. of Chem. Soc. of London, xvii, 421.
=Frankland=, E.—Loc. cit. xix, 372.
=Madan=, H. G.—Loc. cit. xxiii, 22.
=Council= Chem. Soc. of London.—Instructions to Abstractors
from Current Publications. (1879.) Chem. News. xlvii, 15.