What is the main characteristic of an acid when dissolved in water?

The main characteristic of an acid when dissolved in water is that it produces hydrogen ions. This process is fundamental to the definition of acids in the Brønsted-Lowry theory, which states that an acid is a substance that can donate protons (H⁺ ions). When an acid, such as hydrochloric acid (HCl), dissolves in water, it dissociates to release hydrogen ions (H⁺) into the solution, which then contributes to the acidity of the solution.

The presence of hydrogen ions is what gives acids their characteristic properties, such as a sour taste and the ability to turn blue litmus paper red. This ionization is central to understanding the behavior of acids in various chemical reactions and their role in different contexts, such as biological systems and industrial applications.