What is the primary characteristic of acids?

Acids are defined primarily by their ability to increase the concentration of hydrogen ions (H+) in a solution. When an acid is dissolved in water, it releases H+ ions, which contributes to the acidic properties of the solution. This increase in H+ ions is what characterizes an acid and is pivotal in understanding its behavior in chemical reactions.

While the other options might seem related to acids, they do not capture the essence of what makes a substance an acid. For example, while acids do decrease the concentration of hydroxide ions (OH-) when they dissociate, this is more of a secondary effect rather than a defining characteristic. The notion of taste can vary, and not all acids have a bitter taste; in fact, many acids, like citric or acetic acid, taste sour. Finally, the ability to turn blue litmus paper red is a typical property of acids, as it indicates the presence of H+ ions, but it is another result of their acidic nature rather than the primary characteristic itself. Hence, the increase in hydrogen ion concentration in solution is the most direct and defining trait of acids.