Question

What is the Arrhenius definition of acid?

Answer 1

Acids are compound having a pH below 7. They are capable of increasing the hydrogen ion concentration due to which their pH value becomes less than 7. There are various theories that define an acid according to the property of its dissociation in any solution. The Arrhenius is one such theory that looks at the type of ions produced by dissociation of acids.

Complete answer:
Dissociation is the property of any substance to liberate the respective ions in any solution. According to the Arrhenius concept of the acids and bases. An acid is a substance that has the ability to give a proton (hydrogen ion) $\left( {{H}^{+}} \right)$ in any solution. While bases are the substances that liberate the hydroxide ion$\left( O{{H}^{-}} \right)$ in any solution.
When an acid dissociates and gives $\left( {{H}^{+}} \right)$ ions, these ions are so small that they reacts with the water to form ${{H}_{3}}{{O}^{+}}$ ion called hydronium ions, which defines the acidic nature. The extent to which the acids dissociate to give hydrogen ions is determined by the dissociation constant of an acid denoted by ${{K}_{a}}$ that is responsible for the strength of acids. This means the larger the value of ${{K}_{a}}$ stronger the acid will be.
Hence, Arrhenius theory defines an acid to be a substance that has the ability to liberate protons $\left( {{H}^{+}} \right)$or hydrogen ions in any solution.

Note:
Arrhenius theory has its limitations as it considers a species with proton and hydroxide group to be an acid and a base respectively. But most of the acids and bases do not contain a proton or a hydroxide group. Example,$N{{H}_{3}},N{{a}_{2}}C{{O}_{3}},CaO$ are bases without any OH group, and$S{{O}_{2}},S{{O}_{3}},N{{O}_{3}}$ are acids without any hydrogen ions. This can be overcome as the treatment of these acids and bases with water will liberate hydrogen and hydroxide ions.