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Assuming that this is so, the chemical changes enumerated above may be represented in equations as follows:

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(3) Zn + H2SO,Aq=ZnSO, Aq + 2H ;
Fe+ H2SO Aq = FeSO Aq + 2H;
2Al + 3H2SO2Aq = Al,3SO2Aq + 6H;
Mg + H2SO Aq = MgSO1Aq + 2H;
Cd + H2SO Aq= CdSO ̧Âq + 2H ;
Ba + H2SO,Aq = BaSO4 + 2H + Aq.
(4) ZnO + H2SO,Aq= ZnSO,Aq + H2O;
FeO + H2SO Aq = FeSO,Aq + H2O
A1,0 + 3H SO, Aq = A13SO, Aq + 3H2O;
MgO + H2SO Aq = MgSO1Aq + H ̧0;
CdO+H2SO Aq = CdSO2Aq + H ̧Ó

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2

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2

2

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;

BaO+ H2SO Aq = BaSO4 + H2O + Aq.

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The compounds of zinc, iron, &c. produced in these reactions are called salts. If the composition of each of these salts is compared with that of the sulphuric acid, H2SO,, by the interaction of an aqueous solution of which with a metal or the oxide of a metal the salt is produced, it is seen that the salt is composed of a metal together with all the sulphur and oxygen which were combined, before the chemical change began, with hydrogen, forming the compound H2SO,. The compound H2SO, is called an acid. The solution of this acid in water has a sour taste; turns blue litmus red; reacts with zinc, iron, aluminium, and many other metals, to produce salts, and hydrogen; and reacts with oxides of zinc, iron, aluminium, magnesium, and other metals, to produce salts and water.

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The oxide SO, is an acidic, or acid-forming, oxide; that is to say, it reacts with water to produce a compound which is characterised by the properties enumerated in the preceding sentence.

Many oxides resemble sulphur trioxide in that they react with water to produce compounds of oxygen, hydrogen, and the other element of the oxide, which compounds have a sour taste, turn blue litmus red, and interact with metals and metallic oxides to produce salts. These oxides are placed in one class and are called acid-forming, or acidic, oxides. Thus, the oxides whose compositions are represented by the formulae PO, CrO, NO,, SeO,, respectively, are acidic oxides. The interactions of these oxides with water may be thus represented in equations:

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Let us now turn to the other oxide-MgO. Magnesium 125 oxide is a white solid; it dissolves in a large quantity of water; this solution has not a sour taste; it turns red litmus blue. Magnesium oxide interacts with sulphuric acid, and with other acids, to produce salts and water (s. par. 124).

Many oxides resemble magnesium oxide in that they interact with acids to form salts; some of these oxides further resemble magnesium oxide in being more or less soluble in water and thus forming solutions which turn red litmus blue.

Those oxides which interact with acids to produce salts are placed in one class and are called basic oxides, or sometimes salt-forming oxides. Those basic oxides which easily dissolve in water producing liquids which turn red litmus blue are usually placed in a sub-class to which the name alkaline, or alkali-forming, oxides is given. Thus the oxides Al ̧O,, ZnO, CdO, FeO, BaO are basic oxides (s. reactions represented in equations in par. 124).

The oxides of boron, chlorine, iodine, nitrogen, phosphorus, 126 selenion, sulphur, and several other elements, are acidic oxides.

The oxides of aluminium, barium, beryllium, cadmium, copper, iron, lithium, magnesium, mercury, nickel, palladium, silver, sodium, and many other elements, are basic oxides.

Some of the oxides of chromium, molybdenum, tin, tungsten, uranium, vanadium, and a few other elements, are basic, while other oxides of the same elements are acidic.

Can we classify the hydrides by a method similar to that 127 by which we have roughly arranged the oxides in classes?

The only element which forms many compounds with hydrogen is carbon. Some of the hydrides, other than those of carbon, interact with water to produce acids; among these are HBr, HCl, HF, HI, HS. One or two hydrides interact with water to produce compounds which again react with acids to form salts; the best marked hydride of this class is ammonia, NH2. Several hydrides are either unchanged by water, or dissolve in it without producing either an acid or a salt-forming compound; e.g. H,Sb, H.As, H Cu,, H ̧P, H‚Te.

Hydrides cannot therefore be wholly classified by arranging them as acidic (or acid-forming), and basic (or salt-forming),

M. E. C.

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hydrides; but nevertheless several hydrides belong to one or other of these classes.

But there may of course be other properties of the hydrides on which a classification might be based.

We might, for instance, classify the hydrides by looking to the composition of their reacting weights, and arranging them in classes according as the reacting weight is composed of one, two, or more, combining weights of hydrogen, and one, two, or more, combining weights of the other element. Thus the formula MH, would represent the composition of all the hydrides, where H represents one combining weight of hydrogen, M one combining weight of the element other than hydrogen, and x and y vary.

The following table represents a classification of the greater number of the hydrides, excepting those of carbon, framed on this basis.

Hydrides; MH.

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OH2, SeH,, SH,, TeH ̧.
MH.
"MH..
SiH. Cu2H2, OH.

This classification is evidently based altogether on composition. Before deciding whether it is, or is not, a good classification we should study the properties of the hydrides, with the view of finding whether those placed in each class, MH, MH,, &c., are marked by some common property which cuts them off from those in each of the other classes.

129 A slight examination of the hydrides in the foregoing table shews that many are gaseous at ordinary temperatures and pressures; some of these are easily decomposed when mixed with air or oxygen and heated. A classification might be based on the study of these decompositions. In most cases the products of the decomposition are oxide of hydrogen (water; H,O), and oxide, or oxides, of the other element. The following table summarises this method of classifying gaseous hydrides.

Gaseous Hydrides; MH,
Easily decomposed, by mixing with
oxygen and heating, into MO,
and H2O.

SeH,, SH,, TeH.;
SbH, AsÍ ̧, PH;
SiH; Cu,H,,

Not easily decomposed by mixing with oxygen and heating.

BrH, CIH, FH, IH; NH,

This classification is based on the chemical properties of the bodies classified. Before deciding whether it is, or is not, a good classification we should examine the composition of the hydrides, with the view of determining whether those placed in one class have similar compositions.

A comparison of this with the preceding table shews a certain connexion between composition and readiness or unreadiness to interact with oxygen.

The classification which we made of oxides into acidic and 130 basic oxides was based, for the most part, on chemical properties.

Let us now look a little to the compositions of the basic and the acidic oxides. As all are oxides, that is compounds of oxygen each with one other element, it is evident that one of the circumstances which conditions the basic or acidic character of an oxide is the nature of the element with which the oxygen is combined.

What then are the chemical properties of those elements which combine with oxygen to produce basic oxides? And what are the chemical properties of the elements which combine with oxygen to produce acidic oxides?

In par. 9 the decomposition of water by the electric current was described. The products of the electrolysis of water are hydrogen and oxygen; the hydrogen is always produced in contact with the terminal of the wire in connexion with the zinc plate of the battery, and the oxygen is always produced in contact with the terminal of the wire in connexion with the carbon, or copper, or platinum, plate of the battery. These terminals are called the negative electrode (wire coming from zinc plate), and positive electrode (wire coming from copper plate), respectively. As 'electricities of opposite sign attract each other,' hydrogen is called an electro-positive element, and oxygen an electro-negative element.

If hydrogen chloride (HCl) is electrolysed, hydrogen separates at the negative, and chlorine at the positive, electrode : chlorine is therefore said to be an electro-negative element. If hydrogen sulphide (H,S) is electrolysed, sulphur separates at the positive electrode; sulphur is therefore said to be an electro-negative element. Sulphur, chlorine, and oxygen are electro-negative elements: if a compound of sulphur and chlorine is electrolysed sulphur separates at the positive electrode; sulphur is therefore negative relatively to chlorine, and chlorine is positive relatively to sulphur, although, as we

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have seen, it is negative relatively to hydrogen. Electrolysis of a compound of sulphur and oxygen results in the separation of sulphur at the negative electrode; sulphur is therefore positive relatively to oxygen, although it is negative relatively to chlorine. The terms electro-positive and electro-negative are therefore purely relative terms; an element A may be positive towards an element B, but it may be negative towards another element C.

The following list indicates the arrangement of the commoner elements in electrical order; each element is positive to all that precede it and negative to all that come after it: the order is only approximately correct. Negative; O, S, N, F, CI, Br, I, P, As, B, C, Sb, Si, H, Pt, Hg, Ag, Cu, Bi, Sn, Pb, Co, Ni, Fe, Zn, Mn, Al, Mg, Ca, Ba, Sr, Na, K, Rb, Cs, ; Positive.

Taking the elements on the negative side of hydrogen as a class it is found that they vary much in physical properties; some are gases, some are solids, one (bromine) is a liquid; their colours vary much; some are lustrous, some are not; those which are solids are more or less brittle, none is malleable, ductile, or tenacious; they are bad conductors of heat and electricity; their emission-spectra are generally very complex. Some of these elements interact with steam to produce oxygen and a hydride of the element used (s. par. 105 where the reaction of steam with the negative element chlorine is described). The elements relatively positive to hydrogen more nearly resemble each other in many physical properties; most of them are white, or grey, lustrous solids; very many are malleable; several are tenacious and ductile; they are generally hard, and many of them are heavy; they are good conductors of heat and electricity; their emission-spectra, as a rule, are less complex than those of the negative elements. Many of these elements interact with water or steam to produce hydrogen and an oxide of the element used (s. par. 105 where the reaction of steam with iron, and of water with sodium, is described).

The members of the first class are usually called electronegative elements, and those of the second class electro-positive. Hydrogen occupies a position between these classes, but it is more closely related to the positive than to the negative elements.

The physical characters of the positive elements are summed up in the word metal, the physical characters of the negative elements are expressed by the term non-metal.

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