Al′kali, an old chemical term used to denote soluble caustic hydroxides. The alkalies proper are potash, soda, lithia, rubidium and cæsium hydroxides and ammonia. Potash is called the vegetable alkali, soda the mineral alkali and ammonia the volatile alkali. Lime, magnesia, baryta and strontia are called alkaline earths, because they have some of the properties of alkalies. That which especially distinguishes an alkaline substance is the power it has of turning a vegetable blue, green; or a vegetable yellow, reddish brown. Alkalies belong to a general class of substances called bases, which are oxides of metals (usually combined with water), or compounds containing carbon and nitrogen called organic bases, all of which unite with acids to form salts. Alkalies and acids neutralize each other, and the usual caustic or bitter taste of the alkali and the sour taste of the acid usually disappear when a salt is formed. A familiar example is the addition of soda (alkali) to sour milk (acid), which neutralizes the acid, or destroys the sour taste. An alkali also unites with oil or fat to make soap.
