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Nevertheless each of the compounds in the third column (KHSO, &c.) is called a salt.

So far as composition goes we may at present regard a salt as a compound of a metal with the elements of an acid except the whole or a part of the hydrogen of that acid; as regards mode of formation we may say, that a salt is one of the products of the interaction between an acid (in solution) and a metal, a basic oxide, or an alkali.

The meaning of the term salt then evidently includes the meanings of the terms acid, metal, basic oxide, and alkali; and any one of these terms can be understood only by considering the meanings of them all.

We already know the general characteristics of those elements which are called metals; we also know that most basic oxides are oxides of metals; and that alkalis are produced by the interaction of certain basic oxides with water.

The meaning given above to the term salt implies that an acid is a compound of hydrogen. But all compounds of hydrogen are not acids. The characteristic of those compounds of hydrogen which are acids is that when they interact with metals, basic oxides, or alkalis, they exchange the whole or a part of their hydrogen for metal, and thereby form a salt, or salts. Alkalis also are compounds of hydrogen with oxygen and a metal. The composition of a reacting weight of the alkalis is represented by the formula MOH; where M = Li, Na, K, Rb, or Cs. These elements, Li, &c. are the most positive of all the elements.

If the composition of acids is compared with the properties of the element or elements which combine with hydrogen to produce these acids, it is found that the non-hydrogenous constituents of acids, as a rule, are distinctly negative elements. This is shewn by comparing the compositions of the following well known acids with the arrangement of elements in electrical order given in par. 131.

HŇO, H, SO,, HCN, H.C.O., HC103, H,PO,, HI, HCI, HF, H,SiF, H,B,0,, H Sió,

But the relations between the compositions and properties of acids, alkalis, and salts cannot be properly elucidated until a later stage.

The term base is frequently used to include basic oxides, alkaline hydroxides, and alkalis; as thus used it indicates a compound which interacts with an acid to produce a salt and water.

Looking back for a moment we see that in attempting 141 to classify oxides we have been led to classify very many elements. We have divided these elements into electropositive and electro-negative. With the characters expressed by each of these terms we have connected several other physical, and some chemical, characteristics.

The electro-positive elements, as a class, are metallic; their oxides are basic; the oxides of the more electro-positive elements interact with water to form compounds of the composition MOH which are alkalis. Many of these elements decompose water, or steam, with production of hydrogen and a metallic oxide.

The electro-negative elements, as a class, are non-metallic; their oxides are generally acidic, that is they interact with water to produce acids. Some of these elements decompose steam with production of oxygen and a non-metallic hydride.*

* For a fuller treatment of the subject of acids and salts s. Chap. XI.

CHAPTER X.

CHEMICAL NOMENCLATURE.

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BEFORE proceeding further with the examination of the properties of classes of elements and compounds with the view of tracing connexions between changes of composition and changes of properties we must acquaint ourselves with the system of nomenclature used in chemistry.

Many names of elements, and such names of classes of compounds as oxides, hydrides, &c. have been incidentally employed.

A name is given to each element. Sometimes the name expresses a characteristic chemical or physical property of the element; e.g. oxygen=acid-producer, hydrogen=water-producer, bromine, because of its powerful and obnoxious smell (Bpwuos), iodine, because of the violet colour of its vapour (iwdns), chromium, because of its many-coloured compounds (xowna). Sometimes the name is that which was used by the ancients or is a modification of this name; e.g. arsenic (đporevikov), copper (cuprum). Sometimes the name is derived from the name used by the alchemists, many of which were derived from the names of the planets; e.g. mercury. The names of many recently discovered elements are derived from the names of the minerals from which they were first obtained, or from the names of the districts, or in some cases countries, in which these minerals were found; thus strontium (from the mineral strontianite found near the village of Strontian in Argyleshire), beryllium (from the mineral beryll), ytterbrium, yttrium, erbium (from Ytterby the district in Sweden where the minerals were found from which the three elements were obtained), gallium, germanium (the former was discovered by a French, the latter by a German, chemist).

Some names are purely fanciful; e.g. tellurium, selenion, uranium, vanadium, (from tellus = the earth, denun = the moon, the planet Uranus, and the Scandinavian deity Vanadis, respectively). The names of the more recently discovered metals all end in um.

The name given to a compound expresses the qualitative 144 composition of that compound; if more than one compound of the same elements is known, names are given indicative of the relative quantities of the elements which unite to form reacting weights of the compounds.

The name of every compound of two elements ends in ide. Thus all compounds of oxygen with one other element are called oxides. The variety of oxide is indicated by prefixing the name of the element united with oxygen; thus we have iron oxides, zinc oxides, sulphur oxides, &c.

Similarly we have sulphides, i.e. compounds of sulphur with one other element; chlorides

, i.e. compounds of chlorine with one other element; bromides, fuorides, hydrides, &c. &c. We say hydrogen oxide, hydrogen chloride, hydrogen sulphide; not oxygen hydride, chlorine hydride, sulphur hydride: oxygen, chlorine, and sulphur, are all more negative, or more nonmetallic, elements than hydrogen. The name of the more negative of the two elements of a binary compound is changed into a qualifying term ending in ide. Thus it is better to say oxide of chlorine-or chlorine oxide—than chloride of oxygen (or oxygen chloride).

When two oxides, chlorides, &c. of one element are known, the general custom is to form an adjective from the name of the element other than oxygen, &c. and to modify the termination of this adjective so as to express that one compound is composed of more, or less, oxygen, relatively to a fixed mass of the other element, than the other compound is. A comparison of the following names with the composition of each compound as expressed by its formula will illustrate this method of naming pairs of oxides, &c. of the same element. Iron oxides

Sulphur oxides
Ferrous oxide Feo

Sulphurous oxide so,
Ferric Fe,Oz:

Sulphuric
Chromium chlorides

Tin bromides
Chromous chloride CrCl, Stannous bromide Sn Br.
Chromic

CrCl.
Stannic

Sn Br
Copper sulphides
Cuprous sulphide Cus
Cupric

Cus.

»

SO:

2

2

3

Bi,

NO
NO

The termination -ous always indicates less of the nonmetallic or negative element than the termination -ic, relatively to the same mass of the metallic or positive element.

When more than two oxides, &c. of the same element are known, two methods of naming are adopted. Four oxides of bismuth are known. Their compositions, and the names given to each, are as follows :

Hypobismuthous oxide; or Bismuth dioxide Bi, o Bismuthous oxide; or Bismuth trioxide

Bi 0 Hypobismuthic oxide; or Bismuth tetroxide

Bi o
Bismuthic oxide; or Bismuth pentoxide
Five oxides of nitrogen are known :-
Nitrous oxide; or Nitrogen monoxide NO
Nitric oxide

NO
Nitrogen trioxide.
Nitrogen tetroxide

Nitrogen pentoxide NO One oxide gets the termination-ic; another, with relatively less oxygen, the termination -ous; the prefix hypo- is used to express relatively less oxygen than that of the -ic or -ous oxide. The prefix per- is sometimes employed to designate that oxide of a series which has relatively most oxygen.

Or the number of combining weights of oxygen in a reacting weight of each oxide is expressed by the prefixes mono, di, tri, &c. Unfortunately neither system is very strictly carried out. We shall have further examples of each system as we proceed.

The composition of one oxide of a series is sometimes expressed by a name formed from the names of other two oxides of the series ; thus FeO is ferrous, Fe,, is ferric, and Fe,, is ferroso-ferric, oxide.

The name sesquioxide is frequently used; it implies that a positive element and oxygen are united in the ratio of 1:13 combining weights ; thus Fe,O, is often called iron sesquioxide, CrO, chromium sesquioxide, &c.

Acidic oxides are sometimes named so as to indicate the acid obtained by interaction of each with water; thus SO, is called sulphuric anhydride, because it interacts with water to produce sulphuric acid. On this system of naming, the term anhydride means an acidic oxide. Lastly an oxide, chloride, &c. is sometimes distinguished from another oxide, chloride, &c. of the same element by a term indicating some prominent

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