Acid-base theory explains how substances interact based on their ability to donate or accept protons. According to the Brønsted-Lowry theory, an acid is a substance that donates a proton (H⁺), while a base accepts a proton. This interaction can lead to the formation of new substances, affecting the pH level of a solution. Another important concept is the Lewis theory, which defines acids as electron pair acceptors and bases as electron pair donors. This broader definition allows for a wider range of chemical reactions to be classified as acid-base reactions, expanding our understanding of chemical behavior.