ACIDS. In popular language, acids are substances of a corrosive nature, with a sour taste when diluted sufficiently to lose their cor rosive action on the tongue, capable of turning certain blue vegetable coloring matters, such as litmus, to a red, dissolving metals and forming neutral compounds with alkalies. They are classified generally into two the inor ganic and the organic, referring to their origin in the mineral kingdom, or in the vegetable and animal kingdoms. Inorganic acids are rarely found as such in nature, but usually in combinations. A small quantity of nitric and nitrous acids is often present in the atmosphere after thunderstorms, carbonic acid is found in limited extent and over limited areas and hy drochloric and sulphurous acids are detected in the fumes from some volcanic fissures. On the other hand, organic acids are freely distributed throughout the vegetable world—as in all fruits, and to a less degree in the animal kingdom. In modern chemistry an acid is re garded as a salt of hydrogen in which one or more of the hydrogen atoms are replaceable by metallic atoms or by organic radicals. For ex ample, hydrochloric acid (HCI) brought into contact with sodium hydroxide (NaOH) seizes upon the sodium, and releases the hydrogen atom — forming sodium chloride (NaCl) and water (H.,0). An acid containing one such atom of replaceable hydrogen is called mono basic ; if it has two such atoms of hydrogen it is called • dibasic or bibasic; if three, tribasic; and so on. Hydrochloric acid, HCI, is a familiar example of a monobasic acid; it has only one atom of hydrogen that can he replaced by potas sium (for example), with the formation of the single compound 1CCI. Sulphuric acid, H2S0., is a familiar dibasic acid; with potassium it forms the two compounds HKSO. (hydrogen potassium sulphate), and 1C2S0. (normal or basic potassium sulphate?. Phosphoric acid, HiP(/, is a tribasic acid in which one, two or all three of the hydrogen atoms may be re placed by metals or radicals. In a polybasic acid the hydrogen atoms need not necessarily all he displaced by the same element or radi cals; thus microcosmic salt is a phosphate of hydrogen, sodium and ammonium, with the formula HNa(NHOPO4+41110. Adds may be
formed synthetically by uniting hydrogen with non-metallic substances — as with chlorine, to form hydrochloric acid.
When an acid contains oxygen it is com monly named for the substance that is present with the oxygen and hydrogen in the acid. For example, nitric acid is named for nitrogen, and phosphoric acid for phosphorus. It often hap pens that the same element forms more than one acid with oxygen and hydrogen. In these cases it is usual to give the termination -ic to the one in which the oxygen is present in its high est valency, or combining proportion; and the termination when in its next lower valency. For example, H2S0. is called sulphuric acid, while }LSO, is called sulphurous acid. Hypo is used as a prefix where the oxygen is in still lower proportion — as hyposulphurous acid. if the acid contains no oxygen it has the prefix hydro hydrochloric acid (HCI). When an acid has an unusually large oxygen com ponent it has the prefix per —as perchloric acid. When an acid has been deprived of all its water component it becomes an acid an hydride. The salts formed by acids ending in -w have the ending -ate, such as the acid sul phate of potassium, produced by substituting the metal potassium for one of the hydrogen atoms of sulphuric acid, while those formed by acids ending in -ous have the ending -ite, as the sulphite of sodium, and the hypophosphite of calcium. Salts are considered by some chem ists to be acids in which the hydrogen atom has been replaced by the metals. Organic acids are oxides in the second degree of alcohols and aldehydes, combined with a hydrocarbon. They are distinguished by the presence of the car boxyl group —0011:1, in which the hydrogen atom is replaceable by metals, resulting in salts. When the hydrogen is replaced by alkyl radicles esters are formed. The relative strength of various acids is determined by saturating them with a metallic hydroxide. The proportion taken up by each acid is the measure of its relative strength.