A compound is classified as an acid based on the following characteristic:
A) It dissolves into Hydrogen ions in water (H+ and H3O+).
Acids are substances that, when dissolved in water, increase the concentration of hydrogen ions (H+) or hydronium ions (H3O+). This is the defining property of acids according to the Arrhenius and Bronsted-Lowry definitions of acids.
Option B is incorrect because acids have a pH lower than 7, not higher. Option C describes bases, as they produce hydroxide ions (OH-) in water. Option D is not a defining characteristic of acids, as there are acids that can be mixed with oil.