Primary Object of Chemistry
in Chapter 1 of Principles of Chemistry
The primary object of chemistry is the study of the homogeneous substances of which all the objects of the universe are made up, with the transformations of these substances into each other, and with the phenomena which accompany such transformations. Every chemical change or reaction, as it is called, can only take place under a condition of most intimate and close contact of the reacting substances, and is determined by the forces proper to the smallest invisible particles (molecules) of matter. We must distinguish three chief classes of chemical transformations.
- Combination is a reaction in which the union of two substances yields a new one, or in general terms, from a given number of substances, a lesser number is obtained. Thus, by heating a mixture of iron and sulphurs a single new substance is produced, iron sulphide, in which the constituent substances cannot be distinguished even by the highest magnifying power. Before the reaction, the iron could be separated from the mixture by a magnet, and the sulphur by dissolving it in certain oily liquids ; 7 in general, before combination they might be mechanically separated from each other, but after combination both substances penetrate into each other, and are then neither mechanically separable nor individually distinguishable. As a rule, reactions of direct combination are accompanied by an evolution of heat, and the common case of combustion, evolving heat, consists in the combination of combustible substances with a portion (oxygen) of the atmosphere, the gases and vapours contained in the smoke being the products of combination.
Reactions of decomposition are cases the reverse of those of combination, that is, in which one substance gives two or, in general, a given number of substances a greater number. Thus, by heating wood (and also coal and many animal or vegetable substances) without access to air, a combustible gas, a watery liquid, tar,'and carbon are obtained. It is in this way that tar, illuminating gas, and charcoal are prepared on a large scale. All limestones, for example, flagstones, chalk, or marble, are decomposed by heating to redness into lime and a peculiar gas called carbonic anhydride. A similar decomposition, taking place, however, at a much lower temperature, proceeds with the green copper carbonate which is contained in natural malachite. This example will be studied more in detail presently. Whilst heat is evolved in the ordinary-reactions of combination, it is, on the contrary, absorbed in the reactions of decomposition.
The third class of chemical reactions, where the number of reacting substances is equal to the number of substances formed, may be considered as a simultaneous decomposition and combination. If, for instance, two compounds A and B are taken and they react on each other to form the substances C and D, then supposing that A is decomposed into D and E, and that E combines with B to form C, we have a reaction in which two substances A, or D E, and B were taken and two others C, or E B, and D were produced. Such reactions .ought to be placed under the general term of reactions of ' rearrangement,' and the particular case where two substances give two fresh ones, reactions of ' substitution.' S Thus, if a piece of iron be immersed in a solution of blue vitriol (copper sulphate), copper is formed or, rather, separated out, and green vitiriol (iron sulphate, which only differs from the blue vitriol in that the iron has replaced the copper) is obtained in solution. In this manner iron may be coated with copper, so also copper with silver ; such reactions are frequently made use of in practice.
Mendeleev's Periodic Principles