197

Similarly, the chromates and dichromates react with concentrated solutions of acids to form chromium salts and oxygen; e.g.

The salts of chromium, manganese, and iron,—¿.e. compounds derived from acids by replacing hydrogen by chromium, manganese, or iron,-form two series the compositions of which are represented by the general formulæ MX and, M.3X, respectively, where M Cr, Mn, or Fe, and X=2NO,, 2ČIO, SO, SO, ČO„, PO, AsO &c.

The salts MX are called chromous, manganous, and ferrous, salts; those of the composition M,3X are called chromic, manganic, and ferric salts. A few examples of each class of salts are given :

the ferrous

198

Many iron salts of both series are known; salts are all fairly readily oxidised to ferric salts.

Most of the

known manganese salts belong to the manganous class; the manganic salts are all readily reduced to manganous salts. Very few chromous salts have been prepared; they are all easily oxidised to chromic salts.

Chromium and manganese resemble the halogen elements and the elements of the sulphur group in that each forms at least one acidic oxide. The resemblance between chromium and manganese and the sulphur group of elements is further shewn by the compositions of the salts obtained by the interactions of these acidic oxides with basic, or alkali-forming, oxides. Thus (M = Ba, Pb, Cá, K„, Na„, Ag,, &c.);—

The resemblance between manganese and the halogens is well shewn by comparing the compositions of permanganates and perchlorates. Thus ;

Permanganates.

Oxide. not isolated

Acid.

Salts.

Perchlorates.

Oxide. not isolated

H,Mn,O,Aq: known only in aqueous Acid. H2Cl208

solution

MMn2Og

Salts. MC1,08

But chromium and manganese resemble the alkali-metals in that each forms at least one basic oxide.

If we tabulate the compositions, and indicate the pro- 199 perties, of several oxides which have now been examined, we shall find a distinct connexion between these compositions and properties.

The name peroxide is here used to indicate an oxide which reacts with acids to evolve oxygen, and at the same time to form salts which correspond in composition with an oxide with less oxygen than the specified peroxide. (s. reactions of Mn,O̟ and MnO, with sulphuric acid; par. 196.)

3

CrO3

MnO

200

201

202

The elements potassium and sodium are very positive; they are soft, light, solids. They interact with cold water to form hydroxides and hydrogen. The elements chlorine and iodine are very negative: one is a gas, the other a lustrous, fairly heavy, solid. They interact with steam at high temperatures to form hydrides and oxygen. The elements chromium, manganese, and iron are neither very positive nor very negative; they are hard, heavy, malleable, solids. They interact slowly with steam at fairly high temperatures to produce oxides and hydrogen. The lower oxides of the three elements whose properties are intermediate between the very positive and the very negative groups are basic; the highest oxide of one of these elements is distinctly acidic, and the oxide MnO is also acidic although less distinctly so than CrO,. But although CrO, and MnO, are acidic, yet they are not wholly acidic; in their interactions with concentrated acids they exhibit basic properties; although neither forms a corresponding salt, yet both produce salts when they react with acids.

3

The oxide of a very positive element, then, appears to be always basic, even when it is composed of relatively much oxygen with relatively little of the positive element. The oxide of a

very negative element appears to be always acidic, even when it is composed of relatively little oxygen with relatively much of the negative element. The oxide of an element which is neither very positive nor very negative appears to be only basic when it is composed of relatively little oxygen, but acidic, with basic tendencies, when it is composed of much oxygen combined with a relatively small quantity of the other element.

Of the members of the chromium group of elements, chromium forms the most markedly acidic oxide. Chromium has the smallest combining weight of the three elements. Manganese however also forms well marked manganates and permanganates. Considering that the differences between the combining weights of the three elements are very small, we might expect that ferrates, salts analogous in composition to chromates and manganates, would be produced if the proper conditions could be realised. Could we oxidise Fe,Ọ, in contact with a large quantity of a strong alkali, we might expect a higher oxide of iron to be formed and simultaneously to react with the alkali and produce a salt.

2 3

If ferric oxide (Fe,O) is suspended in very strong warm potash solution and chlorine is passed into the liquid, a portion

of the ferric oxide dissolves and a red coloured liquid is obtained. Similarly if ferric oxide is mixed with a large quantity of solid potash and a little potassium nitrate, the mixture is melted, kept molten for a time, cooled, and dissolved in water, a red coloured liquid is produced. These red liquids evolve oxygen, and precipitate ferric oxide, when heated. By measuring the oxygen evolved and determining the ferric oxide precipitated, conclusions can be drawn as to the composition of the compound in the original red solution. Such measurements have been made; they are in keeping with the hypothesis that the red liquids contain potassium ferrate, K FeO. This salt appears to exist only in solution in much potash. The change which occurs when this solution is heated is probably as follows;—

2K ̧FeO +x KOHAq + 2H2O = (x + 4)KOHAq + Fe ̧0 ̧ + 30.

4

2 3

Although no acidic oxide of iron has been isolated, yet we may say that the formation of potassium ferrate shews that a compound of iron with much oxygen would be an acid-forming oxide.

Chromium and manganese must then be classed both with 203 the metallic and with the non-metallic elements. A consideration of their physical properties alone would lead us to place them in the class metals; a consideration of the chemical properties of their lower oxides would confirm this conclusion; but a consideration of the chemical properties of their higher oxides shews that the elements in question are fairly closely related to the undoubtedly non-metallic elements sulphur and chlorine.

The properties of oxides are evidently conditioned by the 204 chemical characters of the elements with which oxygen is combined, and also by the ratio of the numbers of combining weights of oxygen and the other element which are united in a reacting weight of each oxide. The properties of hydroxides are also conditioned by the chemical characters, and by the relative masses, of the elements which are combined in a reacting weight of each hydroxide.

As we advance in our study of classes of elements and compounds we shall find that a similar statement holds good for each class of compounds.

Our examination of the properties and compositions of 205 classes of compounds has shewn that such terms as basic oxide or acidic oxide are relative. The oxide CrO, is acidic in its reactions with water and alkalis, but it is basic in its

206

reactions with concentrated acids: the oxide MnO, is acidic in its reactions with alkalis, but not with water; it is basic in its reactions towards concentrated acids. It is only the oxides of the very positive elements which are basic, and the oxides of the very negative elements which are acidic, in all their reactions. But even in these cases, the terms basic and acidic imply that the interactions of the oxides with other compounds (acids and alkalis) have been examined.

Chemistry is not the study of elements and compounds alone, but it is the study of the interactions of elements and compounds.

The importance of chemical classification is so great that an examination of another group of elements will be made before we pass to other parts of our subject.

The elements nitrogen, phosphorus, arsenic, antimony, and bismuth are placed in the same class.

Nitrogen and bismuth are the only elements of the class which occur uncombined with others in nature. Oxides and sulphides of the other elements, or compounds of these with other compounds, are found in several rocks; none of the elements except phosphorus and nitrogen occurs very widely distributed or in very large quantities. The elements arsenic, antimony, and bismuth are obtained by reducing their oxides by heating with finely powdered charcoal. Phosphorus is obtained by heating calcium phosphate (Ca2PO) with charcoal.

207 These elements all combine directly with oxygen and the halogens, and all except nitrogen combine directly with