Final answer:
Arrhenius' theory was modified because it only explained acid-base reactions in aqueous solutions and could not account for reactions in non-aqueous solvents or explain the behavior of certain weak bases like ammonia.
Step-by-step explanation:
The Arrhenius theory had to be modified because it failed to explain the behavior of non-aqueous solutions. Specifically, the Arrhenius theory defined acids as substances that increase the concentration of H+ ions in aqueous solutions and bases as substances that increase the concentration of OH- ions.
This definition was limited to aqueous (water) solutions and could not account for acid-base behavior in non-aqueous solvents, such as benzene.
Furthermore, Arrhenius' theory did not explain the acid-base properties of substances that do not release H+ or OH- ions in water, such as ammonia (NH3), which acts as a weak base by releasing hydroxide ions in the presence of water despite not containing OH- itself.
The limitation of the Arrhenius concept to aqueous solutions and its inability to describe the behavior of all acids and bases necessitated the development of new theories, including the Brønsted-Lowry theory, which expanded the definition of acids and bases beyond the presence of aqueous H+ and OH- ions.