Editing Acid (section)

===Arrhenius acids===
[[File:Arrhenius2.jpg|thumb|150px|Svante Arrhenius]]

In 1884, [[Svante Arrhenius]] attributed the properties of acidity to hydrogen cations (H<sup>+</sup>), later described as [[Proton#Hydrogen ion|protons]] or [[Hydron (chemistry)|hydron]]s. An '''Arrhenius acid''' is a substance that, when added to water, increases the concentration of H<sup>+</sup> ions in the water.<ref name="Oxtoby8th"/><ref name="Ebbing"/> Chemists often write H<sup>+</sup>(''aq'') and refer to the hydrogen cation when describing acid–base reactions but the free hydrogen nucleus, a [[proton]], does not exist alone in water, it exists as the '''hydronium ion''' (H<sub>3</sub>O<sup>+</sup>) or other forms (H<sub>5</sub>O<sub>2</sub><sup>+</sup>, H<sub>9</sub>O<sub>4</sub><sup>+</sup>). Thus, an Arrhenius acid can also be described as a substance that increases the concentration of [[hydronium]] ions when added to water. Examples include molecular substances such as hydrogen chloride and acetic acid.

An Arrhenius [[base (chemistry)|base]], on the other hand, is a substance that increases the concentration of [[hydroxide]] (OH<sup>−</sup>) ions when dissolved in water. This decreases the concentration of hydronium because the ions react to form H<sub>2</sub>O molecules:

:H<sub>3</sub>O{{su|p=+|b=(aq)}} + OH{{su|p=−|b=(aq)}} ⇌ H<sub>2</sub>O<sub>(liq)</sub> + H<sub>2</sub>O<sub>(liq)</sub>

Due to this equilibrium, any increase in the concentration of hydronium is accompanied by a decrease in the concentration of hydroxide. Thus, an Arrhenius acid could also be said to be one that decreases hydroxide concentration, while an Arrhenius base increases it.

In an acidic solution, the concentration of hydronium ions is greater than 10<sup>−7</sup> [[Mole (unit)|moles]] per liter. Since pH is defined as the negative logarithm of the concentration of hydronium ions, acidic solutions thus have a pH of less than 7.