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

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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,, 2C1O,, SO, SO, CO, 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 :

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the ferrous

Most of the

Many iron salts of both series are known; salts are all fairly readily oxidised to ferric salts. 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, Ca, K, Na,, Ag,, &c.);

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


Oxide. not isolated




Oxide. not isolated

H,Mn,O,Aq: known only in aqueous Acid. H2Cl2Og



Salts. MC1208

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,O, and MnO, with sulphuric acid; par. 196.)

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Do not form

acids; but interact with oxides to form salts.




Do not form alkalis,

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Yer in sts of both series are MAYA Any really oxidised to ferri INNY LANAse sits belong to the m LARIN MN are all readily reduced ry NY ALOs salts have been pre Arase to chronic salts.

Chromen and manganese resemble t and the elements of the sulphur group in just one scidic oxide. The resemblance bet manganes and the sulphur group of elemen by the compositions of the salts obtained by these acidic oxides with basic, or alkali-form (M=R, PA, C, K, Na, Ag, &c.);

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of all, except bismuth, are known. Several
lement, except bismuth, are known. The
re basic, one is feebly acidic; the oxides
timony are acidic, but also slightly
nitrogen and phosphorus are acidic.
least one oxychloride. Several salts-
obtained by replacing hydrogen of the
ents-of bismuth, and a few salts of
re known. Neither nitrogen nor phos-
f these elements, interacts with acids

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he more important compounds of the 208
group may be expressed by the
M= N, P, As, Sb, or Bi.

known when M = Bi).

03, M2O,, M2O,.

MX, (X = Cl, Br, I, F.).
OX, MOX, and many complex

M6ÑO, M3SO, M.3C0,:

ly known when M = Bi.

M= N, P, As, or Sb, are colour- 209
ures and pressures.

mony is formed when hydrogen
compound of arsenic or anti-
sulphuric acid,-

2ASH, +3H,0 + 6ZnSOA

ed and part reacts with the

is generally prepared by nium chloride) with lime;


ined by a similar process

KIAq+ H2O.

NH, and PH, combine in properties to the salts

- MHI &c. (M = N or P).




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.

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,O, tact 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.

in con

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

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