Which of the following is NOT classified as a strong acid?

Hydrofluoric acid (HF) is indeed not classified as a strong acid, and this is the reason it is the correct answer in this context. Strong acids are typically defined by their ability to completely dissociate into their ions in an aqueous solution.

HF is known to be a weak acid because, although it does dissociate to some extent, it does not undergo complete ionization in water. This means that in a solution, equilibrium is established, and a significant amount of HF remains undissociated, causing it to behave as a weak acid with relatively low acidity compared to strong acids.

In contrast, the other acids listed—perchloric acid (HClO4), sulfuric acid (H2SO4), and nitric acid (HNO3)—are classified as strong acids because they completely dissociate in water. Each of these acids donates protons (H⁺ ions) easily, resulting in higher concentrations of H⁺ ions in solution and a lower pH, which characterizes strong acids.

Thus, recognizing the full dissociation characteristics of these compounds allows you to clearly differentiate HF as a weak acid in comparison to the others which exhibit strong acidity.