Which of the following is a strong acid?

The correct answer is HI (hydroiodic acid), which is classified as a strong acid. Strong acids are those that completely dissociate into their ions in aqueous solution, meaning that they release all of their hydrogen ions (H⁺) when dissolved in water. HI is one of the seven strong acids, which also include HCl, HBr, HNO₃, H₂SO₄, HClO₄, and HClO₃.

The characteristics of strong acids are important for understanding their behavior in chemical reactions and in solution. Since HI fully dissociates, it produces a high concentration of hydrogen ions, leading to a low pH and strong acidic properties.

In contrast, H₃PO₄ (phosphoric acid) is a weak acid because it only partially ionizes in solution. NH₃ (ammonia) is not an acid at all; rather, it functions as a weak base. CH₃COOH (acetic acid) is also a weak acid, only partially dissociating into acetate ions and hydrogen ions in solution.

Understanding the classifications of acids is fundamental in chemistry, especially when analyzing reactions and predicting the behavior of substances in various conditions.