Which statement best describes a solution that contains unreacted ions?

A solution that contains unreacted ions is typically in a state where the ions present do not actively participate in any additional reactions that would affect the pH of the solution. When unreacted ions are present, they are often at equilibrium, meaning they are stable and not engaging in any significant chemical reactions with other substances in the solution.

In this context, if the pH remains unchanged upon adding a base, it indicates that the existing ions are not reacting with the added base to form new products. This stability of the solution’s pH suggests that the concentration of hydrogen ions (H⁺) and hydroxide ions (OH⁻) remains consistent despite the introduction of a base. Consequently, the presence of unreacted ions essentially buffers the solution, preventing significant changes to its pH.

Therefore, the statement that the solution's pH is unchanged accurately describes the situation of unreacted ions, as the ions present do not lead to a reaction that would alter the acidity or basicity of the solution. This understanding reflects the behavior of equilibrium systems in aqueous solutions and the effect of additional species on that equilibrium.