How can you tell if a compound is an acid?

There are many different definitions for acid

. I'll assume that you want to use the simplest and most commonly used definition, and that you are only interested in aqueous solutions.

Dissolve a pure sample of the compound in freshly distilled water. If the concentration of hydrogen ions goes up, the compound is an acid.

There are several ways to detect the rise in hydrogen ion concentration. Certain dyes called indicators change color when exposed to acid solutions; for example, blue litmus turns pink. You can also use a pH meter; a pH of lower than 7 corresponds to an acidic solution. (pH goes down as hydrogen ion concentration goes up...)

There is no easy way to decide whether a compound is an acid from its molecular formula alone. Usually, molecular formulas of acids are written with the ionizable hydrogen first; for example, acetic acid can be written as HC₂H₃O₂ to indicate that one hydrogen ion can be released per molecule.

Author: Fred Senese senese@antoine.frostburg.edu

General Chemistry Online! How can you tell if a compound is an acid?