This phenomenon suggested that acid–base reactions are reversible-that is, that the products of the reaction can interact to regenerate the starting material. It also soon became clear that many of these displacements could take place in either direction according to experimental conditions. In addition, it was found quite early that one acid could be displaced from a salt with another acid, and this made it possible to arrange acids in an approximate order of strength. The absence of any apparent physical basis for the phenomena concerned made it difficult to make quantitative progress in understanding acid–base behaviour, but the ability of a fixed quantity of acid to neutralize a fixed quantity of base was one of the earliest examples of chemical equivalence: the idea that a certain measure of one substance is in some chemical sense equal to a different amount of a second substance. In spite of their imprecise nature, these ideas served to correlate a considerable range of qualitative observations, and many of the commonest chemical materials that early chemists encountered could be classified as acids (hydrochloric, sulfuric, nitric, and carbonic acids), bases (soda, potash, lime, ammonia), or salts (common salt, sal ammoniac, saltpetre, alum, borax). ![]() Bases (or alkalies) were characterized mainly by their ability to neutralize acids and form salts, the latter being typified rather loosely as crystalline substances soluble in water and having a saline taste. Other properties associated at an early date with acids were their solvent, or corrosive, action their effect on vegetable dyes and the effervescence resulting when they were applied to chalk (production of bubbles of carbon dioxide gas). The English word acid, the French acide, the German Säure, and the Russian kislota are all derived from words meaning sour (Latin acidus, German sauer, Old Norse sūur, and Russian kisly). Acids were probably the first of these to be recognized, apparently because of their sour taste. The idea that some substances are acids whereas others are bases is almost as old as chemistry, and the terms acid, base, and salt occur very early in the writings of the medieval alchemists.
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