A Brønsted acid is a substance that can donate a proton (H⁺) to another substance in a chemical reaction. This definition is part of the Brønsted-Lowry theory, which focuses on the transfer of protons between molecules. When a Brønsted acid donates a proton, it becomes a Brønsted base, which can accept a proton in return. Common examples of Brønsted acids include hydrochloric acid (HCl) and sulfuric acid (H₂SO₄). These acids are often used in various chemical reactions, including neutralization reactions, where they react with Brønsted bases to form water and a salt.