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The sulphides of phosphorus, arsenic, and antimony, are therefore acidic in their reactions with alkalis and alkaline sulphides; sulphide of bismuth is not acidic, it interacts with acids to form salts and hydrogen sulphide, just as oxide of bismuth interacts with acids to form salts and hydrogen oxide. The following formulae exhibit the relations of composition between (1) acidic oxides and their oxyacids, (2) basic oxides and their salts, (3) acidic sulphides and their sulphoacids, and (4) basic sulphides and the salts obtained by their reactions both with acids and with acidic sulphides.
A classification of the elements of the nitrogen group based on the properties of their sulphides would place phosphorus, arsenic, and antimony, with the non-metals, and bismuth with the metals. Nitrogen would be placed with the non-metals, but to some extent apart from the other members of the group.
We have now learned something of the methods used in chemistry for classifying elements and compounds. We cannot classify elements without at the same time classifying compounds; we cannot arrange compounds in groups without at the same time arranging elements in groups.
As class-marks we have used, the properties and the composition of oxides; the properties and the composition of sulphides; the formation, composition, and interaction with water, of haloid compounds; the formation, or non-formation, the properties, and the composition, of hydrides; the production of compounds with oxygen and hydrogen which are either
acids, alkalis, or intermediate between these; the occurrence or non-occurrence of allotropic change; the position of elements in the electrical series; &c., &c.
We have traced the change from one class of compounds to another accompanying the increase or decrease in the relative mass of one of the elements in a series of binary or ternary compounds. We have seen basic oxides combining with oxygen and forming acidic oxides; we have compared basic hydroxides with acidic hydroxides of the same element. We have also traced the change from one class of compounds to another accompanying the substitution of a positive by a negative element in compounds of similar composition. We have compared acidic hydroxides of negative elements with alkaline hydroxides of positive elements, and both with neutral hydroxides of elements which were neither markedly positive nor strongly negative.
We have also learned something of the conditions under which classes of compounds are formed. We have seen some elements combining directly at ordinary temperatures with oxygen; others only at high temperatures; others only when they interact with compounds under such conditions that oxygen is produced in the interactions.
Of each compound we have asked; of what elements, and of how much of each, is a reacting weight of this compound composed? What can this compound do, and under what conditions does it perform its chemical functions? Composition, and properties, these have been our guides.
In examining the classification of elements and compounds we have travelled far from the starting point of our inquiries. We have not looked much to the general conditions of chemical change, or to the circumstances which modify the course and the final goal of chemical processes. We have been content to know that acids, for instance, are compounds of hydrogen with one or more negative elements, generally with oxygen and another more or less negative element, and that these compounds interact with metals, with basic oxides, and with alkalis, to produce salts. We have not asked; under what conditions does this or that acid interact? if two acids react with one alkali will one combine with the whole of the alkali, or will the alkali be divided between the acids? But we must now come back to such inquiries as these.
CONDITIONS WHICH MODIFY CHEMICAL CHANGE.
BODIES interact chemically only when brought into very close contact. The contact may be effected by dissolving, gasefying, or melting, one or more of the reacting substances; or, in some cases, by submitting the substances to very great pressure.
Barium chloride and sodium sulphate, for example, remain unchanged when mixed; but when aqueous solutions of these compounds are mixed, barium sulphate and sodium chloride are at once produced. Ammonia and hydrogen chloride gases combine to produce ammonium chloride. A mixture of iron and sulphur remains unchanged for an indefinite time; but when iron filings are added to molten sulphur chemical change occurs and iron sulphide is formed. The same product is formed by exposing the mixture of finely divided iron and sulphur to a pressure of several thousand tons on the square inch.
As a general rule chemical change proceeds more rapidly between a solid and a liquid, or a solid and a gas, the more finely divided the solid is, that is, the greater the surface exposed by a given quantity of the solid. Thus a piece of iron oxidises slowly in ordinary air; but very finely divided ironobtained by strongly heating iron tartrate in a glass tube and closing the tube while hot-oxidises so rapidly in ordinary air that the particles of the oxidising iron glow and emit light. Granulated zinc, that is zinc in thin irregular-shaped pieces, dissolves in dilute sulphuric acid much more quickly than a compact mass of zinc.
If one or more of the possible products of a chemical interaction is gaseous under the experimental conditions, that
chemical interaction usually occurs readily. Thus calcium carbonate (CaCO) rapidly interacts with dilute hydrochloric acid solution, to produce calcium chloride (CaCl) which remains in solution, and carbon dioxide (CO) which escapes as a gas;
CaCO3 + 2HClAq = CaCl, Aq + H2O + CO2.
Calcium carbonate is decomposed by heat to solid calcium oxide and gaseous carbon dioxide: CaCO, (heated) CaO + CO,. Zinc and dilute sulphuric acid interact at ordinary temperatures to produce zinc sulphate and gaseous hydrogen :
Zn + H2SO, Aq= ZnSO, Aq + 2H.
If one or more of the possible products of a chemical inter- 230 action between solutions is a solid under the experimental conditions, that chemical change usually occurs readily. Thus aqueous solutions of barium chloride and sodium sulphate interact when mixed to produce solid barium sulphate, and sodium chloride which remains in solution :
BaCl, Aq+ Na,SO,Aq = BaSO, + 2NaClAq.
A solution of antimony chloride in hydrochloric acid interacts with water to produce solid antimony oxychloride, and hydrochloric acid which remains in solution:
SbCl, (in HClAq) + H ̧O + xH ̧O = SbOCl + 2HClAq + xH ̧O.
Potassium acetate is soluble in alcohol, potassium carbonate is insoluble in alcohol; if carbon dioxide is passed into an alcoholic solution of potassium acetate, the formation of an insoluble compound becomes possible; this compound, potassium carbonate, is formed and precipitated. On the other hand, if acetic acid is added to an aqueous solution of potassium carbonate, the formation of a gas becomes possible; this gas, carbon dioxide, is formed and the carbonate is decomposed. These reactions furnish an instance of the reversal of a chemical change by alterations in the experimental conditions such that in one case the formation of a solid, and in the other the formation of a gas, becomes possible.
The conditions which chiefly modify chemical changes, besides those already mentioned, are temperature, and the relative masses of the interacting substances.
Influence of temperature on chemical change. A 231 chemical change occurs within a definite range of temperature. Some changes take place readily and completely at ordinary temperatures; other changes begin only at higher temperatures.
Thus sodium or potassium oxidises rapidly in ordinary air ; iron filings are oxidised rapidly only by heating in oxygen. Iron and dilute sulphuric acid readily react to produce iron sulphate and hydrogen; copper and sulphuric acid do not interact until the temperature is raised to 100° or more. Hydrogen and oxygen do not combine until the temperature is raised very considerably.
232 The products of a chemical interaction sometimes vary according to the temperature at which the substances are caused to interact. Thus sodium chloride and sulphuric acid react at ordinary temperatures to produce sodium-hydrogen sulphate and hydrogen chloride, but at higher temperatures the chief product, besides hydrogen chloride, is sodium sulphate; the two reactions may be represented thus:
(1) 2NaCl + 2H2SO1 = 2NaHSO4 + 2HCl ;
(2) 2NaCl + H2SO1 = Na2SO + 2HCl.
A solution of bismuth iodide in hydriodic acid interacts with cold water to precipitate bismuth iodide; BiI, (in HIAq) + xH2O (cold) = BiI ̧ + HIAq + xH2O. But the same solution interacts with hot water to precipitate bismuth oxyiodide;
BiI, (in HIAq) + H2O+xH2O (hot) = BiOI + 2HIAq + ×H2O. A cold aqueous solution of copper sulphate reacts with cold caustic potash solution to precipitate copper hydroxide; a hot aqueous solution of copper sulphate reacts with hot caustic potash solution to precipitate copper oxide:
(1) CuSO Aq (cold) + 2KOHAq (cold) = CuO,H ̧ + K„SO̟ ̧Aq.
(2) CaCO, (heated strongly) = CaO + CO2,
Ammonia and hydrogen chloride gases combine when mixed to form ammonium chloride; but ammonium chloride is completely resolved into ammonia and hydrogen chloride at