Acids

There are certain acid properties which were noted early in the history of chemistry. According to the Arrhenius acid-base concept, a substance is classified as an acid if it ionizes to form hydrogen(+) ions in aqueous solution. For example, hydrochloric acid reacts with water to form hydrogen ions which are transferred to a water molecule to form a hydronium ion.

Other systems classify substances as acids if they act as proton donors (Bronsted-Lowry theory) or as electron-pair acceptors (Lewis theory). These two classification methods are not limited to solutions in water.

Common Acids

Some common molecules which are classified as acids.

Acid Strength

An acid which ionizes almost completely in solution is said to be a strong acid, wheras one which has a small degree of ionization is a weak acid. Of the common acids, hydrochloric acid, nitric acid and sulfuric acid are strong acids. Acetic acid is a weak acid.

The acidity of solutions is measured in terms of the pH of the solution, and strong acids will lower the pH more for a given molarity of solution.