What defines an acid in a solution?

An acid in a solution is defined as a substance that increases the hydrogen ion concentration. This definition is grounded in the Brønsted-Lowry theory of acids and bases, which posits that acids are proton donors. When an acid dissolves in water, it dissociates to release hydrogen ions (H⁺) into the solution, resulting in an increase in the hydrogen ion concentration.

Characteristic properties of acids, such as their ability to taste sour, conduct electricity in solution, and react with metals to produce hydrogen gas, reinforce this definition. The presence of more hydrogen ions in a solution indicates a lower pH, which is typical of acidic solutions. Thus, the increase in hydrogen ion concentration is a fundamental characteristic that clearly identifies a substance as an acid.