Final answer:
An Arrhenius acid is a substance that increases the concentration of hydrogen ions in aqueous solution when dissolved, forming hydronium ions.
Step-by-step explanation:
An Arrhenius acid can be defined as a compound that, when dissolved in water, increases the concentration of hydrogen ions (H+), also known as protons. The presence of hydrogen ions is key to classifying a substance as an Arrhenius acid. When H+ ions are present in water, they combine with water molecules to form hydronium ions (H3O+). This is often represented in chemical equations by showing H₂O on the reactants' side along with the acid, to balance the equation properly. Classic examples of Arrhenius acids include hydrochloric acid (HCl), which ionizes in water to give H+ and Cl- ions.