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Similarly, the chromates and dichromates react with concentrated solutions of acids to form chromium salts and oxygen; e.g. (1) 2K CrO, Aq+5H SO
2k 50, Aq+Cr,380,Aq +5H,0 + 30. (2) K,Cr,0,Aq + 4H SO,
*K,80 Aq+Cr.380 Aq+44,0 + 30. The salts of chromium, manganese, and iron,-i.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,, 210, SO., SO2, CO2, ķPO,, AsOq, &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 : -ous salts.
-ic salts. Ferrous sulphate
Ferric sulphate Fe,3804 Manganous sulphate MnSO4 Manganic sulphate Mn,3804 Chromous sulphate CrS04 Chromic sulphate Cr,3804 Ferrous nitrate Fe2NO3 Ferric arsenate Fe,2Aso Manganous chlorate Mn2ció: Manganic phosphate Mn,2P04 Chromous oxalate CrC,04 Chromic selenite Cr,3SeOz.
Many iron salts of both series are known; the ferrous salts are all fairly readily oxidised to ferric salts. Most of the kņown 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
Oxide. not isolated Oxide. SO,
Acid. not isolated Acid. H,ŠO4
The resemblance between manganese and the halogens is well shewn by comparing the compositions of permanganates and perchlorates. Thus ;Permanganates.
Perchlorates. Oxide. not isolated
Oxide. not isolated Acid. H,Mn,03Aq: known only in aqueous Acid. H,C1,08
solution Salts. MMn,08
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. (8. reactions of Mn, 0, and Mno, with sulphuric acid ; par. 196.)
1. Alkali. forming.
been isolated, or are isolable
C1,0. C10 . 1,05
3. Form acids with water.
Mn304. Mn,03 . MnO, . Croz.
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 Croz But although Cro, and Mn0, 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.
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,., 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.
If ferric oxide (Fe0g) is suspended in very strong warm potash solution and chlorine is passed into the liquid, a portion
of the ferric oxidė dissolves and a red coloured liquid is
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
reactions with concentrated acids: the oxide MnO, is acidic
It is only
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.
about 6000 points.
45° (data un. about 4500 about 2700
odourless, like, solid: crystalline, very brittle,
solid. brittle, solid.
These elements all combine directly with oxygen and the
fied at very