According to the Arrhenius theory, how is an acid defined?

In the context of the Arrhenius theory, an acid is specifically defined as a substance that increases the concentration of hydrogen ions (H+) in an aqueous solution. This definition highlights the fundamental behavior of acids in water, where they dissociate to release H+ ions, resulting in an increase in the acidity of the solution.

This theory focuses on the role of ions in chemical reactions, particularly in aqueous environments. The presence of more H+ ions directly correlates with a lower pH, indicating a more acidic solution. By contrast, other options do not accurately capture the essence of how acids behave according to this theory. For instance, the notion of decreasing hydroxide ions pertains more to the characteristics of bases, while the other choices either describe neutralization processes or involve unrelated reactions that do not define acids under the Arrhenius framework.