According to the Brønsted-Lowry theory, what characterizes an acid?

In the context of the Brønsted-Lowry theory, an acid is defined as a substance that donates protons (H+) during a chemical reaction. This theory expands the concept of acids and bases beyond the classical definitions, which were primarily focused on hydrogen ion concentration and hydroxide ions. According to Brønsted and Lowry, when an acid donates a proton, it typically forms its conjugate base.

For instance, when hydrochloric acid (HCl) is dissolved in water, it donates a proton to a water molecule, resulting in the formation of hydronium ions (H3O+) and chloride ions (Cl-). This transfer of protons is the fundamental characteristic that identifies a substance as an acid under this theory.

The other concepts related to acids, such as increasing the pH or forming salts when reacting with bases, are important in the broader context of acid-base chemistry but do not specifically define what qualifies as an acid according to the Brønsted-Lowry framework. Therefore, the defining feature that sets an acid apart is its ability to donate protons.