According to the Arrhenius definition, what is an acid?

The Arrhenius definition of an acid is centered around the behavior of substances in water. According to this theory, an acid is specifically identified as a substance that increases the concentration of hydrogen ions (H⁺) when dissolved in water. When an Arrhenius acid, such as hydrochloric acid (HCl), dissociates in water, it releases hydrogen ions and contributes to the acidity of the solution. This increase in hydrogen ions directly correlates with a decrease in pH, making the solution more acidic.

This definition is critical in distinguishing acids from other substances. For example, hydroxide ion producers are categorized as bases, not acids. Though acids can react with metals in specific reactions—such as producing hydrogen gas—the definition is not exclusive to this behavior and does not encompass all substances that might lower pH. Therefore, the precise identification of an acid according to the Arrhenius definition is rooted exclusively in its ability to produce hydrogen ions in an aqueous solution.