Arrhenius Acid Definition and Examples

pH strip next to sliced orange

An Arrhenius acid is a substance that dissociates in water to form hydrogen ions or protons. In other words, it increases the number of H+ ions in the water. In contrast, an Arrhenius base dissociates in water to form hydroxide ions, OH-.

The H+ ion is also associated with the water molecule in the form of a hydronium ion, H3O+ and follows the reaction:

acid + H2O → H3O+ + conjugate base

What this means is that, in practice, there aren't free hydrogen cations floating around in aqueous solution.

Rather, the extra hydrogen forms hydronium ions. In more discussions, the concentration of hydrogen ions and hydronium ions are considered interchangeable, but it's more accurate to describe hydronium ion formation.

According to the Arrhenius description of acids and bases, the water molecule consists of a proton and a hydroxide ion. The acid-base reaction is considered a type of neutralization reaction where the acid and base react to yield water and a salt. Acidity and alkalinity describe the concentration of hydrogen ions (acidity) and hydroxide ions (alkalinity).

Examples of Arrhenius Acids

A good example of an Arrhenius acid is hydrochloric acid, HCl. It dissolves in water to form the hydrogen ion and chlorine ion:

HCl → H+ (aq) + Cl- (aq)

It's considered an Arrhenius acid because the dissociation increases the number of hydrogen ions in the aqueous solution.

Other examples of Arrhenius acids include sulfuric acid (H2SO4), hydrobromic acid (HBr), and nitric acid (HNO3).

Examples of Arrhenius bases include sodium hydroxide (NaOH) and potassium hydroxide (KOH).