Acid

as'id, a term given early in the history of chemistry to substances that had a sour taste. The name is derived from the Latin acetum, meaning sharp or sour, and was first applied to vinegar to indicate its characteristic property. The meaning of the term was later broadened to include all acid-like substances, and the word acetic was made to apply specifically to the acid of vinegar. Also, a general property of acids is their power to change the color of certain dyes, as, for example, litmus. When litmus is brought into a solution containing an acid, the natural blue color is changed to red. But some acids are neither sour nor have the power to change the color of a dye. These as a rule are insoluble in water and belong to the class generally called "weak" acids. The liberation of hydrogen gas when an acid is brought in contact with certain metals, as zinc or magnesium, is another important characteristic. Alongside of this is also the property of combining with alkalies, such as caustic soda or caustic potash. Chemically, an acid may be defined as a compound containing hydrogen which can be replaced by a metal. Hydrogen is the only essential constituent of all acids. Many other substances, like sugar, kerosene, and alcohol, also contain hydrogen; but not one of them shows all of the properties of an acid. Among the common elements forming acids in combination with hydrogen, or with hydrogen and oxygen, are carbon, chlorine, nitrogen, sulphur, phosphorus, silicon, and arsenic. When free from water, acids do not conduct electricity; dissolved in water they conduct, and are decomposed by, the electric current, hydrogen being liberated. A base is opposite in properties to an acid, as in general it restores the blue color of litmus and destroys or neutralizes the acid. The interaction of an acid with a base forms a distinct compound, called a salt.

There is an enormous number of acids, most of which occur ready-formed in the earth or in plants and animals. Many acids are manufactured products. The common acids are hydrochloric acid, sulphuric acid, and nitric acid. The usual forms are solutions of the acids in water. Hydrochloric acid has a pungent, suffocating odor. The commercial article, known also as muriatic acid, is a solution of the gas, hydrogen chloride, in water. When heated, effervescence ensues, a portion of the gas being driven out. The acid occurs among the gases emitted by volcanoes. It is manufactured from common salt. Nitric acid, sometimes called aqua fortis, is a fuming liquid having a suffocating odor. It colors animal tissue, notably the skin, yellow. So-called fuming nitric acid has a brown-red color. The acid occurs in very small quantity in air, rain water, and spring water. It is manufactured on a large scale from Chile saltpeter. It is used much in the arts in dyeing, in the etching of metals, and in medicine. Sulphuric acid, also called oil of vitriol, is a heavy, oily, strongly corrosive liquid. It has a strong affinity for water, and when mixed with water a large amount of heat is developed. Organic matter is charred, the acid acquiring a brown color. The acid occurs in small amount in volcanic waters. Enormous quantities are manufactured in England and the United States, each country producing upward of a million tons annually. The method of manufacture is complicated and consists in bringing sulphur dioxide, made by roasting pyrites, and nitric acid in contact with air and steam in large lead chambers. Sulphuric acid is more commonly used than any other acid and for a greater variety of purposes; for example, in refining petroleum, in the manufacture of fertilizers, in bleaching and dyeing, in the production of coal-tar dyes, and, in conjunction with nitric acid, in the manufacture of nitroglycerin and guncotton. Among the acids occurring chiefly in plants and animals, the so-called organic acids, the best known is acetic acid. This acid is a colorless, clear liquid of strongly acid reaction. Almost all of the acetic acid used in the industrial arts is made by the dry distillation of wood. Large quantities of the weaker acid are made from dilute alcohol. A dilute solution of the acid, known as vinegar, is made by passing the alcohol in a slow stream through a barrel filled with beech shavings. The fermentation of the alcohol to acid is aided by bacteria, so-called "mother-of-vinegar." Citric acid is found in the lemon, currant, cranberry, and other sour fruits; oxalic acid, in Oxalis (sheep sorrel); malic acid, in sour fruits, especially apples, and in maple sap; formic acid, in red ants, stinging nettles, pine needles, the honey bee, and honey; butyric acid, in rancid butter, Limburger cheese, and sauerkraut; lactic acid, in sour milk and sauerkraut; palmitic acid, in palm oil; tartaric acid, one of the most widely distributed acids, in grapes. The salts of tartaric acid are called tartrates, chief among which is the well known cream of tartar.

JULIUS HORTVET.