Which of the following is an example of a strong acid?

A strong acid is defined as one that completely dissociates into its ions in an aqueous solution. Hydrochloric acid (HCl) is a prime example of a strong acid, as it ionizes fully when dissolved in water, producing a high concentration of hydrogen ions (H⁺). This complete dissociation results in a significant decrease in pH, demonstrating the strong acidic nature of HCl.

In contrast, hydrofluoric acid (HF), formic acid (HCOOH), and phosphoric acid (H₃PO₄) are classified as weak acids. HF only partially dissociates in solution, leading to fewer hydrogen ions and a relatively less acidic solution. Similarly, HCOOH and H₃PO₄ do not dissociate completely, with H₃PO₄ actually being a polyprotic acid that can donate more than one proton but still does not do so completely in each step.

Therefore, the identification of HCl as a strong acid emphasizes its complete ionization, setting it apart from the others listed, which do not exhibit the same behavior in solution.