Which of the following substances releases positively charged hydrogen ions (H+) into water?

The correct answer is that an acid releases positively charged hydrogen ions (H+) into water. When an acid is dissolved in water, it ionizes and produces hydrogen ions, which are responsible for the acidic properties of the solution. This process affects the pH level of the solution, making it lower because higher concentrations of hydrogen ions correlate to a more acidic environment.

In contrast, bases typically accept hydrogen ions or release hydroxide ions (OH-) when dissolved in water, which is not the behavior associated with acids. Salts are the result of the neutralization reaction between acids and bases, and they can dissociate into cations and anions in a solution but do not specifically release hydrogen ions. Alkalis, a subset of bases that dissolve in water, also do not release hydrogen ions but instead contribute to the formation of hydroxide ions. Therefore, the characteristic behavior of acids in releasing hydrogen ions clearly distinguishes them from other substances.