Alkali
The word "alkali" is derived from Arabic "al qalīy" (or "alkali"), meaning "the calcined ashes" (see calcination), referring to the original source of alkaline substances. A water-extract of burned plant ashes, called potash and composed mostly of potassium carbonate, was mildly basic. After heating this substance with calcium hydroxide ("slaked lime"), a far more strongly basic substance known as "caustic potash" (potassium hydroxide) was produced. Caustic potash was traditionally used in conjunction with animal fats to produce soft soaps, one of the caustic processes that rendered soaps from fats in the process of saponification, one known since antiquity. Plant potash lent the name to the element potassium, which was first derived from caustic potash, and also gave potassium its chemical symbol K (from the German name Kalium), which ultimately derived from alkali.
Acid–base reaction
This causes the protonation of water, or the creation of the hydronium (HO) ion. Thus, in modern times, the symbol H is interpreted as a shorthand for HO, because it is now known that a bare proton does not exist as a free species in aqueous solution.
The Arrhenius definitions of acidity and alkalinity are restricted to aqueous solutions, and refer to the concentration of the solvent ions. Under this definition, pure HSO and HCl dissolved in toluene are not acidic, and molten NaOH and solutions of calcium amide in liquid ammonia are not alkaline. This led to the development of the Bronsted-Lowry theory and subsequent Lewis theory to account for these non-aqueous exceptions.