2. a substance that yields hydrogen ions in solution and from which hydrogen may be displaced by a metal to form a salt. For the various acids, see under the specific name, such as
acetic acid. All acids react with bases to form salts and water (neutralization). Other properties of acids include a sour taste and the ability to cause certain dyes to undergo a color change. A common example of this is the ability of acids to change litmus paper from blue to red.
Inorganic acids are distinguished as
binary or
hydracids, and
ternary or
oxyacids; the former contain no oxygen; in the latter, the hydrogen is united to an electronegative element by oxygen. The hydracids are distinguished by the prefix
hydro-. The names of acids end in
-ic, except in the case in which there are two degrees of oxygenation. The acid containing the greater amount of oxygen has the termination
-ic, the one having the lesser amount has the termination
-ous. Acids with the termination
-ic form the salts ending in
-ate; those ending in
-ous form the salts ending in
-ite. The salts of hydracids end in
-ide. These rules are demonstrated by the acids and salts: hydrochloric acid (HCl), sodium chloride (NaCl), sulfuric acid (H
2SO
4), sodium sulfate (Na
2SO
4), sulfurous acid (H
2SO
3), sodium sulfite (Na
2SO
3). Acids are called
monobasic, dibasic, tribasic, and
tetrabasic, respectively, when they contain one, two, three, or four replaceable hydrogen atoms.
The most common organic acids are carboxylic acids, containing the carboxyl group (-COOH); examples are acetic acid, citric acid, amino acids, and fatty acids. Their salts and esters end in
-ate, e.g., ethyl acetate. Other organic acids are phenols and sulfonic acids.
Acids play a vital role in the chemical processes that are a normal part of the functions of the cells and tissues of the body. A stable balance between acids and bases in the body is essential to life. See also
acid-base balance.