AFFINITY.—Chemical affinity or chemical attraction is the force by which the union takes place of two or more elements to form a chemical compound. According to another definition, it is a force exerted between two or more bodies at an infinitely minute distance apart, by which they give rise to a new substance having different properties to those of its component parts. Elements have the greatest affinity for other elements which differ most in their chemical properties. Thus, hydrogen has a great affinity for chlorine and oxygen, but the affinity between chlorine and oxygen is much weaker. Acids unite readily with alkalies and most metals with sulphur. When two salts are mixed together they are decom posed if an insoluble substance can be formed. Thus, (silver nitrate) + NaCI (sodium chloride) yields (sodium nitrate), and insoluble AgC1 (silver chloride) and (barium chloride) + (magnesium sulphate) yields (magnesium chloride) and insoluble (barium sulphate), as (sulphuric acid) expels HC1 (hydrochloric acid) or CO, (carbonic acid), but when two salts are fused and a more volatile compound is formed, it is driven off, as when (ammonium chloride) is heated with dry (calcium carbonate) then NH,),CO, (ammonium carbonate) volatizes. The relative affinity between different substances
varies with their temperature, insolubility, and power of vaporization. Alteration of the temperature alters the affinity. Thus, if mercury be heated to its boiling point it absorbs oxygen, which it liberates at a still higher temperature.
The investigation of the relative proportions in which bodies combine forms the basis of the atomic theory or doctrine of chemical equivalents.
Affinity of Solution is such an affinity as exists between a soluble salt and its fluid in which it is dissolved. Until the liquid is saturated with the salt the two can combine in an indefinite ratio, instead of being limited to the fixed proportions, in which alone chemical affinity operates. See Chemistry.