ACIDS, its'Idz (Lat. acidus, sour). A large and important class of chemical substances. They all contain hydrogen, part or all of which is replaced by metals when the acids are brought in contaet with metallic hydroxides. The com pounds formed by substituting metals for the hydrogen of acids are termed the salts of those metals, and therefore the acids themselves may be regarded as salts of hydrogen. An example may render these definitions more clearly intel ligible. When the sour principle of vinegar is brought in contact with potassium hydroxide, a reaction ensiles, resulting in the formation of a new substance. A chemical analysis, com bined with a determination of the molecular weight of the sour principle of vinegar, shows that the molecule of the latter must be repre sented by the formula on the other hand, the substance formed with potassium hydroxide is represented by the formula F.vidently, part of the hydrogen of the sour prin ciple of vinegar has been replaced by the metal potassium ( K 1. We therefore class the sour principle of vinegar with the acids (it is the well known acetic acid) ; and we class the substance obtained by its action on potassium hydroxide with the salts (it is called the acetate of potas sium, while acetic acid itself may be called the acetate of hydrogen).
\lost acids have a sour taste and change the blue color of litmus. to red. These properties, however, are not strictly characteristic of acids, silicic acid, for instance, possessing neither, though—like a true acid—it combines with metallic hydroxides to form salts.
According to the maximum number of their hydrogen atoms replaceable by metals acids are termed mono-basic, di-basic, tri-basic, etc. No matter how great the excess of potassium hydrox ide employed, only one hydrogen atom of acetic acid, can be replaced by potassium, the only resulting salt having the formula C,H,K.02. Acetic acid is, therefore, said to be a mono-basic acid. By the action of a limited amount of potassium hydroxide on sulphuric acid (11,S0,) a salt called the acid sulphate of potassium (IIKS0,) may be obtained; this salt is formed by substituting the metal potassium for one of the hydrogen atoms of sulphuric acid. But if
an excess of potassium hydroxide is used, both of the hydrogen atoms of sulphuric acid are replaced by potassium, and the salt known as the neutral sulphate of potassium (K,S0,) is produced. Sulphuric acid is therefore said to he a di-basic acid. In like manner phosphoric acid is found to be a tri-basic acid, etc.
Acids containing carbon among their constit uent elements are called organic acids, because some of them were originally found in the organic world. :Most organic acids are found to contain one or more carboxyl groups (COOH) it is the hydrogen of these groups that is replace able by metals. These acids are called carboxylic acids, and their basicity is determined by the number of carboxyl groups they contain. The carboxylic acids are subdivided into carbocyclic and fatty acids, according as their molecules do or do not contain those rings of which the so-called aromatic benzene-nucleus is the most important. Thus benzoic acid, is a earbocyclic acid; acetic acid, CH,C0011, is a fatty acid. An interesting group of substances belonging to the aromatic series and, like acids, combining with metallic hydroxides, are not included among the true aromatic acids because they do not contain the carboxyl group. These substances, called phenols (q.v.), are found to be weaker than the weakest carboxylic acid known, viz., carbonic acid.
The specific strength of an acid depends, natu rally, on its composition and chemical consti tution. But the precise nature of that relation is as yet unknown. The correctness of the very methods of measuring the strength of acids is, according to some eminent authors, still subject to doubt. It is, however, remarkable and cannot be denied, that the different methods employed yield very nearly coincident results.