Acid-base theories explain how substances interact in chemical reactions. The most common theories include the Arrhenius theory, which defines acids as substances that produce hydrogen ions (H⁺) in water, and bases as those that produce hydroxide ions (OH⁻). Another important theory is the Brønsted-Lowry theory, which defines acids as proton donors and bases as proton acceptors. The Lewis theory expands on these ideas by defining acids as electron pair acceptors and bases as electron pair donors. These theories help scientists understand the behavior of acids and bases in various chemical processes, including neutralization reactions and pH changes.