REACTION, in chemistry, a general term for any chemical change that is brought about by the mutual interaction of two or more sub stances. Reactions between two primary sub stances may be conveniently divided into three classes, as follows : (1) Those in which the two substances combine additively, so as to form a single third substance by their union. (2) Those in which one of the primary sub stances is broken up, the other primary sub stance combining additively with one or all of its parts, so as to produce one or more new substances. (3) Those in which there is a double decomposition of the two primary sub stances, the several parts of the first combining with those of the second so as to produce two or more new substances. The first of these is illustrated by the action of oxygen gas upon metallic sodium, the combination taking place additively according to the equation 2Na + 0 NaaO. The second is illustrated by the action of chlorine gas upon an excess of methane (CH,) ; methyl chloride and hydrochloric acid being formed (upon exposure to diffuse light) in accordance with the equation CH.4+ 20= CH,CI HCI. The third general type of reaction between two primary substances (that, namely, in which there is a double decomposi tion) may be illustrated by the action of hydro chloric acid upon caustic potash, KOH, as indi cated by the equation KOH + HCI = H30 + KCI, the products of the reaction being water and chloride of potassium.
Reactions take place with widely varying degrees of intensity. This intensity is known in chemistry as (affinity» and is evidenced by an exhibition of great heat, violent ebullition, explosion and like physical manifestations.
These phases are more noticeable when the two combining substances are of different tricities;i that is, one positive and the other negative. Temperature has a decided effect upon the velocity with which chemical reactions take place, and many reactions may be re strained by cold. On the other hand, after a reaction has taken place a sufficient rise in tem perature may decompose the new compound into its primary constituents.
In all reactions it is observed that substances combine in certain definite proportions, and these are called their "combining weights? or proportions? The law which is found to obtain in all cases is that the elements combine in the ratios of their combining weights, or in simple multiples of such ratios— the term weight* standing for the smallest relative mass of the element (that is, the smallest number of atoms) in which the element enters into combination with other ele ments. Thus, 110.34 parts of potassium bromide are required to supply the exact amount of bromine to convert 100 parts of silver into silver bromide. Conversely, 100 parts of silver com bined with chlorine will form 132.845 parts of silver chloride — no more and no less, though a much greater quantity of chlorine may be present.
The subject of chemical reaction is inti mately connected with the entire question of chemical constitution, of which much remains to be learned. See the article CHEMISTRY; also CATALYSIS ; CHEMICAL AFFINITY ; EQUILIBRIUM, CHEMICAL; PERIODIC LAW ; VALENCY.