I) Arrhenius theory: An Arrhenius acid is a substance that produces H^+ ions (or H3O^+ ions) when dissolved in water. An Arrhenius base* is a substance that prod

Here are the fundamental concepts for each acid-base theory:

• i) Arrhenius theory: An Arrhenius acid is a substance that produces $\text{H}^+$ ions (or $\text{H}_3\text{O}^+$ ions) when dissolved in water. An Arrhenius base is a substance that produces $\text{OH}^-$ ions when dissolved in water. This theory is limited to aqueous solutions.

• ii) Brønsted-Lowry theory: A Brønsted-Lowry acid is a proton ($\text{H}^+$) donor. A Brønsted-Lowry base is a proton ($\text{H}^+$) acceptor. This theory expands the definition beyond aqueous solutions and introduces the concept of conjugate acid-base pairs.

• iii) Lewis theory: A Lewis acid is an electron pair acceptor. A Lewis base is an electron pair donor. This is the broadest definition, encompassing reactions that do not involve proton transfer or the formation of $\text{OH}^-$ ions, such as reactions involving metal ions or electron-deficient compounds.