In aqueous solutions, the hydrogen ions produced by acids react with water molecules to form hydronium ions (H3O+). Therefore, in terms of H3O+ and OH- ions in water, an acid can be defined as a substance that increases the concentration of H3O+ ions in water, while a base is a substance that increases the concentration of OH- ions in water.

For example, when hydrochloric acid (HCl) is dissolved in water, it ionizes to produce H+ ions and chloride ions (Cl-). The H+ ions then react with water molecules to form H3O+ ions:

HCl + H2O → H3O+ + Cl-

On the other hand, when sodium hydroxide (NaOH) is dissolved in water, it ionizes to produce OH- ions and sodium ions (Na+):

NaOH → Na+ + OH-

The OH- ions produced by the base then react with H3O+ ions in water to form water molecules:

OH- + H3O+ → 2H2O

This reaction results in a decrease in the concentration of H3O+ ions and an increase in the concentration of OH- ions, making the solution basic.

In summary, the Arrhenius theory describes acids as substances that increase the concentration of H3O+ ions in water, while bases increase the concentration of OH- ions in water.