Final answer:
Hydrofluoric acid (HF) is not a strong acid in aqueous solution, unlike the other acids listed (HCl, HBr, HI, HClO₄), which are all strong acids that completely dissociate in water.
Step-by-step explanation:
The student has asked which of the listed compounds is not a strong acid in aqueous solution. The options provided are: HF (hydrofluoric acid), HCl (hydrochloric acid), HClO₄ (perchloric acid), HBr (hydrobromic acid), and HI (hydroiodic acid).
In this list, HF is the one that is not a strong acid. Strong acids such as HCl, HBr, HI, and HClO₄ dissociate almost completely in water, releasing their protons to form hydronium ions (H3O+) and their conjugate bases. However, HF, while still an acid, does not completely ionize in water, which makes it a weak acid.
Hence, the correct answer to the student's question is HF (hydrofluoric acid), which is not classified as a strong acid, but rather a weak one.