An Arrhenius acid is a substance that dissociates in water to form hydrogen ions or protons (H⁺).
For example hydrochloric acid: HCl(aq) → H⁺(aq) + Cl⁻(aq).
An Arrhenius base is a substance that dissociates in water to form hydroxide ions (OH⁻).
For example sodium hydroxide: NaOH(aq) → Na⁺(aq) + OH⁻(aq).