Advanced Placement (AP) Chemistry Practice Exam

What happens to a strong acid in an aqueous solution?

1. It establishes an equilibrium with undissociated molecules.

2. It dissociates partially into ions.

3. It completely dissociates into ions.

4. It forms a concentrated solution.

The correct answer is: It completely dissociates into ions.

In an aqueous solution, a strong acid completely dissociates into ions. This means that when a strong acid, such as hydrochloric acid (HCl), is dissolved in water, it breaks apart entirely into hydrogen ions (H⁺) and chloride ions (Cl⁻). This complete dissociation is a defining characteristic of strong acids, distinguishing them from weak acids, which only partially dissociate. The reason for this behavior lies in the strength of the acid itself. Strong acids have a strong tendency to donate protons (H⁺ ions) to the water, resulting in a solution that consists nearly entirely of ions. This characteristic leads to a high concentration of hydronium ions (H₃O⁺) in solution, thereby contributing to a low pH value, which is typical for strong acids. Thus, the correct answer is supported by the fact that strong acids do not establish significant equilibrium with undissociated molecules, which would imply that some of the acid remains intact in a solution. Instead, they fully dissociate, enriching the solution with ions.