Based on the degree of ionization of an acid in an aqueous solution, acids are classified into strong acids and weak acids. Generally, strong acids are considered to ionize completely in aqueous solutions-examples include hydrochloric acid and nitric acid-whereas weak acids ionize only partially in aqueous solutions, such as acetic acid and carbonic acid.
Strong Acids: Perchloric acid, hydroiodic acid, sulfuric acid, hydrobromic acid, hydrochloric acid, nitric acid, iodic acid, etc.
Moderately Strong Acids: Oxalic acid (ethanedioic acid), sulfurous acid, phosphoric acid, pyruvic acid, nitrous acid.
Weak Acids: Carbonic acid (which some consider a moderately strong acid), citric acid, hydrofluoric acid, malic acid, gluconic acid, formic acid, lactic acid, benzoic acid, acrylic acid, acetic acid, propionic acid, stearic acid, hydrosulfuric acid, hypochlorous acid, boric acid.
Regarding oxyacids: a relatively common form is designated simply as "acid" (e.g., chloric acid); other oxyacids are named based on whether the oxidation state of the central acid-forming element is higher or lower than that of the standard acid, or whether they contain a peroxide -O-O- structure. For example: chloric acid (HClO₃, where the oxidation state of chlorine is +5); perchloric acid (HClO₄, oxidation state +7); chlorous acid (HClO₂, oxidation state +3); and hypochlorous acid (HClO, oxidation state +1). Similarly, compounds such as HSO₅ and H₂S₂O₈, which contain -O-O- bonds, are named peroxymonosulfuric acid and peroxydisulfuric acid, respectively. An acid formed by the condensation of two simple oxyacid molecules with the elimination of one molecule of water is termed a "pyroacid" (or "condensed acid"); alternatively, the prefix "di-" (or "meta-") may be used in the nomenclature. The functional group formed when a simple oxyacid loses all of its hydroxyl groups is called an "acyl group" (specifically, an "oxyacyl group"); for instance, -SO₂ is termed the sulfonyl group, and CrO₂Cl₂ is termed chromyl chloride. If the chemical formula of an oxyacid is written in the general form MOₙ(OH)ₘ (where M represents a metal or nonmetal), the relative strength of common oxyacids can be estimated based on the value of *n*: if *n* = 0, it is a very weak acid (e.g., boric acid, H₃BO₃).
