Svante Arrhenius developed an acid-base theory based on the idea that acids increase the concentration of hydrogen ions (H+) in water while bases increase the concentration of hydroxide ions (OH-). According to his theory, when an acid and base are mixed in water, they neutralize each other by reacting to form a salt and water. Arrhenius provided examples of common acids and bases donating H+ or OH- ions in water to support his theory.