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with sodium or potassium or the hydroxide of either of these metals.

We have thus arrived at a fairly clear notion of the meaning 190 of the term acid so far as the reactions of acids with metals, basic oxides, and alkalis, are concerned.

But what kind of compounds are acid ? What are the compositions of these compounds? Have acids any common composition corresponding to their common property of forming salts under specified conditions ?

The following formulae represent the compositions of several compounds which are acids--

HCI, HBr, HI, HF, H SO, H SO, HNO, HNO . H.CO., H.CO,, HCNO, H, B,0 , HCIO, H.,10, HBrO; HPỎ,,"H PO, ,Mn,0g, HVO, HSbOy, H,860, H Moon, Hw8,, i sno,,7,8,0, H, Seo,, H,Teö,,,Ta o,

These acids are all compounds of hydrogen, and most of them are compounds of hydrogen with oxygen and another element.

Are all compounds of hydrogen with oxygen and another element acids ?

We know that aqueous solutions of the following compounds are alkalis, i.e. exhibit properties strongly opposed to those of acids ;-LiOH, NaOH, KOH, RbOH, CsOH. The following compounds are also alkaline ;-CaO, H,, BaO, H,, Sro,H,, MgO,H,. Therefore all compounds of hydrogen with oxygen and another element are not acids. What then are the general characters of those elements the compounds of which with hydrogen and oxygen are acids? Most of the acids in the foregoing list are compounds of hydrogen and oxygen with non-metallic elements (S, N, C, B, CI, I, Br, P, Se, Te); hence it would appear probable that the union of a non-metallic, or negative, element with hydrogen and oxygen would produce an acid. Further investigation gives a general confirmation to this conclusion.

By far the greater number of the compounds of the non-191 metallic elements with oxygen and hydrogen are acids. Some of the acids in the foregoing list are compounds of hydrogen and oxygen with elements which are usually classed as metals ;-H.Mnog, HM00, H WO,, H,TO,, H Snoz, HVOHS10, H sbó, The elements tungsten, molybdenum, tantalum, vanadium, and antimony exhibit many of the physical, and some of the chemical, properties of metals; but they each form at least one acidic oxide; they do not

form well marked and stable salts by interacting with acids ; they are electro-positive to all the distinctly non-metallic elements (0, S, N, P, CI, Br, I, F, B, C, Si, S, Se), but they are electro-negative to most of the distinctly metallic elements. The elements manganese and tin are undoubtedly metals, both physically and chemically; they interact with acids to form salts; they form basic oxides; they do not combine with hydrogen; they react with steam at high temperatures to produce oxides and evolve hydrogen; they are heavy, lustrous, malleable, solids which conduct heat and electricity fairly well.

The compounds (1) H,MnO, and (2) H Sno, are however acids; the ratios of the numbers of combining weights of hydrogen, metal, and oxygen in a reacting weight of each of these compounds is H:M: 0 in (1)=1:1:4, and in (2) =1:1:11. The compound in a reacting weight of which there is relatively the greater quantity of oxygen (H, Mn,Os) is decidedly an acid; the other compound in a reacting weight of which there is relatively less oxygen is an acid, but at the same time it interacts with concentrated sulphuric acid (and with some other acids) to produce a salt and water ;thus H, Sn0, + 2H,SO, (heated) = Sn(SO2)2 + 3H_0.

Certain compounds of metals with hydrogen and a relatively large quantity of oxygen are then acids.

But in the list of acids given a little further back there appeared compounds of hydrogen with certain non-metallic elements other than oxygen; viz. HCI, HBr, HI, HF. We have already learned something of the elements in question : we know that they are typical non-metals; that they are

strongly electro-negative. 192 It appears then that acids are compounds of hydrogen,

generally with markedly electro-negative or non-metallic elements of which oxygen is usually one; but in some cases with oxygen and another element which, physically and chemically, is neither strongly metallic nor strongly negative; and in a few cases with oxygen and another element which must be placed in the positive or metallic class. In the last mentioned cases there is usually a large quantity of oxygen

combined with hydrogen and the metallic element. 193 We now see that acidic oxides are generally the oxides of the electro-negative or non-metallic elements.

The oxides corresponding to the acids HVO, HSbO, HM00, H.WO,, H Ta,O,, are V,05, Sb0s, M00,, Wo , and Taos, respectively: these acidic oxides are the oxides of elements which are both metallic and non-metallic ; they are all composed of a large quantity of oxygen combined with a comparatively small quantity of the other element. The compositions of these oxides, as regards ratio of oxygen to other elements, may be compared with the compositions of the oxides C1,0, N, O, P,0g, all of which are acidic, and all of which are oxides of strongly negative elements.

Oxides of the less positive metals formed by the union of much oxygen with comparatively small masses of the metals may then be acidic.

The oxides of the very positive elements (Li, Na, K, Rb, Cs, Ca, Sr, Ba, Mg, &c.) are all basic; the oxides of the less positive, but still distinctly positive, elements (Fe, Zn, Ni, Cd, Hg, &c.) are generally basic; the lower oxides —i.e. those with comparatively little oxygen-of the more negative of the metals (Sno, PbO, Pb,og, MnO, Mn,Os, &c.) are usually basic, but the higher oxides of some of these elements (Sno, PbO,, &c.) are feebly acidic.

Chromium is a heavy, hard, lustrous, grey, solid. It con- 194 ducts heat and electricity fairly. This element scarcely reacts with steam; at a high temperature a very little oxide is produced. It interacts with acids to form salts. It forms stable haloid, and also several oxyhaloid, compounds. It does not combine with hydrogen. It is positive to most of the distinctly non-metallic elements, but negative to most of the distinctly metallic elements. From what we have learned concerning basic and acidic oxides, we might expect to find the lower oxides of chromium basic, and perhaps an oxide with comparatively much oxygen, acidic.

The elements iron and manganese resemble chromium in many of their properties. It is advisable now briefly to consider this group of elements. Chromium. Manganese.

Iron. 195 Combining weights

52:4
55

56 Spec. gravities (water=1) about 6.5 about 7 about 7.5 Melting points

above 20000 about 15000 about 15000 Appearance &c. All white, lustrous, solids: chromium and manganese very hard; iron softer; all malleable and fairly ductile; all fair conductors of heat and electricity.

Preparation. All obtained by reducing the oxides M,O2 with finely powdered carbon at a very high temperature.

Chemical properties. The elements decompose steam forming oxides and hydrogen; iron at about 100°, manganese at a M. E. C.

10

196

higher temperature, and chromium at a still higher temperature and then only very slowly. None of these elements combines directly or indirectly with hydrogen. They all interact with oxygen to produce several oxides. The compositions of the chief oxides are expressed by the formulæ MO, M,0, M,O,; the oxides MnO,, CrO2, and Cro, are also known.

The oxides MO are basic; they react with acids to form salts; thus,

MO+H SO Aq = MSO, Aq +H.O. The oxides M,O., where M = Mn or Fe, also react with acids to form salts ; but the compositions of the salts do not correspond to that of the oxides. With dilute sulphuric acid Mn, 0, forms MnSo, and Mn0,; thus

Mn, O. + 2H SO, Aq=Mn0, + 2MnSO Aq + 2H,0. With hot concentrated sulphuric acid MnSo, is formed and oxygen is evolved; thus

MnO, + 3H, SO, = 3MnSo, +3H,0 +0.
The corresponding oxide of iron reacts with sulphuric acid
to form a mixture of the two sulphates of iron ; thus,

Fe,On + 4H SO Aq=FeSO, Aq + Fe 380 Aq +4H,0.
The existence of Cr. O, is doubtful.

The oxides M,Oare also basic; their interactions with
sulphuric acid may be represented thus,
(1) Cr,O, + 3H 80

Aq = Cr,380Aq + 3H,O. (2) Mn,03+H SO Aq = MnO, + MnSO Aq + H,0: (or with hot concentrated sulphuric acid ; Mn, O2 + 2H SO.

= 2MnSo, + 2H,0+ O). (3) FeO, +3H SO, Aq=Fe, 380, Aq+31,0.

But the oxides M,0, are also feebly acidic; salts of the form RO.M,o are known, where RO=KO, CaO, ZnO, &c.

Of the remaining oxides, Cro, has not been fully examined; it seems to react with acids to form the same salts as are obtained. from Cr,0g, and at the same time to evolve oxygen.

MnO, with concentrated acids gives salts of the form MnX (where X = SO, 2NO3, CO2, &c.), and at the same time oxygen is produced ; thus, MnO, +H SO, Aq = MnSO, Aq +H,0+ 0 (the amount of water must be small).

This oxide also reacts with alkalis to form salts the negative part of which is formed of the MnO, and

oxygen ;

thus when this oxide is fused with solid potash the reaction is 3MnO, + 2KOH =K,0. Mno, (=K,Mn0)+H1,0 + Mn,03

The salt potassium manganate (KMnO) dissolves in water, and from the solution other salts are obtained ; thus,

K,MnO, Aq + BaCl, Aq = BaMnO, +2KCIA. These salts, the composition of which is expressed by the formula MMnO, where M = Ba, Ca, K,, Na,, &c., are called manganates. Their composition is similar to that of sulphates (MSO). An aqueous

solution of a manganate reacts with dilute acids to form a permanganate; thus, 3K, MnO, Aq + 2H, SO, Aq=K,MnO,Aq + 2K SO, Aq

+ 2H,0 + MnO A series of permanganates is known,

MMn,,, where M = Ba, Ag,, K, &c. The oxide Cro, dissolves in water to form an acid liquid from which crystals of the acid H Cro, have been obtained. This acid is called chromic acid; it reacts with alkalis and basic oxides to give a series of chromates MCro, (M =K, Na,, Ag,, Ba, Ca, &c.). The composition of the chromates is similar to that of the manganates (MMnO) and sulphates (MSO).

An aqueous solution of a chromate reacts with dilute acids to form a dichromate ; thus

2K,Cr0, Aq+H,SO, Aq=K,Cr,0,Aq + K SO Aq+H,0.

The dichromates, MCr 0,, are not similar in composition to the permanganates (MMno), but they agree in composition with a series of sulphur salts known as di- (or pyro-) sulphates, MS,O

The oxide Cro, also interacts with hot concentrated acids to form salts and oxygen ; thus,

2Cr0, +3H SO, = Cr 350, + 3H,0 + 30. The manganates and permanganates react with acids to form manganese salts and oxygen; thus, (1) K,MnO, Aq+ 2H,SO, Aq

=K_SO, Aq + MnSO, Aq + 2H,0 + 20. (2) K,MnO, Aq + 3H SO,Aq: =K,SO, Aq + 2MnSO, Aq + 3H,0+50.

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