A Brønsted acid is a substance that donates a proton (H⁺) to another substance in a chemical reaction. This process is known as proton transfer. When a Brønsted acid donates a proton, it becomes a Brønsted base, which is a substance that accepts a proton. The Brønsted-Lowry theory of acids and bases helps explain many chemical reactions, particularly in aqueous solutions. For example, when hydrochloric acid (HCl) dissolves in water, it donates a proton to water, forming hydronium ions (H₃O⁺) and chloride ions (Cl⁻). This interaction illustrates the fundamental behavior of Brønsted acids and bases.