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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 M,H, 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 u and y vary.
The following table represents a classification of the greater number of the hydrides, excepting those of carbon, framed on this basis.
Cu H, O, H, 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.
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; HO), and oxide, or oxides, of the other element. The following table summarises this method of classifying gaseous hydrides.
Gaseous Hydrides; M,H, Easily decomposed, by mixing with Not easily decomposed by oxygen and heating, into M,0, mixing with oxygen and and H,O.
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 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 (HS) 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
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; 0, 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.
With these physical characters are associated certain distinct chemical characters; e.g. nature of the products of the reactions of each class of elements with water. Of these chemical characters we shall learn more as we proceed, meanwhile we have to look especially to the chemical properties of the oxides of each class. Looking at the matter broadly we may assert that the oxides of the non-metallic elements are acidic, and the oxides of the metallic elements are basic. Further we may assert that most of the non-metals combine with hydrogen to form hydrides, but that very few hydrides of the metals are known.
We have now associated certain properties of several elements with one definite chemical characteristic of the oxides of these elements.
Non-metallic elements are electro-negative as regards metallic elements; their oxides are generally acidic; most of them form hydrides. Metallic elements are electro-positive as regards non-metallic elements; their oxides are generally basic; few of them form hydrides.
ACIDS AND SALTS.
The sketch of the chemical properties of oxygen and hydrogen in Ch. viii. has shewn that, in order to learn anything of these properties we have been obliged to examine the properties of many of the compounds of these elements, and to do this we have found it necessary to study the properties of very many other elements. Our attempt to classify oxygen and hydrogen has taught us, more fully than before, that the chemical properties of this or that definite kind of matter are the properties exhibited in the interactions of the specified kind of matter with other kinds, both elements and compounds.
We have also learned that a classification of elements carries with it a classification of compounds, and vice versa; and we have found that the method of chemical classification is based on the study both of the composition and the properties of the kinds of matter to be classified.
The study of the chemical properties of hydrogen and oxygen led to a consideration of the meaning of the terms acidic and basic oxides, and this brought with it a consideration of the properties of acids and salts. These classes of compounds are of great importance in the classification of elements and compounds.
We must now consider them somewhat more fully.
Sulphur trioxide (SO2) is a typical acidic oxide; potassium oxide (K,O) is a typical basic oxide. Let each be dissolved in water and each solution be evaporated; the first solution, until it becomes thick; the second, until all the water is removed and a solid remains. Then let the thick oily liquid obtained by evaporating the first solution be cooled to – 10° or so; crystals separate ; let these be collected with suitable precautions and analysed; their composition is expressed by the formula