What characteristic defines HCl as a strong acid?

The defining characteristic of hydrochloric acid (HCl) as a strong acid is that it dissociates completely in solution. When HCl is dissolved in water, it separates into hydrogen ions (H⁺) and chloride ions (Cl⁻) entirely, which results in a high concentration of free hydrogen ions. This complete ionization contributes to its strong acidic properties, as a strong acid is defined by its ability to donate protons (H⁺) to the solution effectively.

This complete dissociation is what leads to the low pH value of the solution, but the pH itself is not the primary criterion that defines the strength of the acid. The boiling point and the reaction rate with metals do not determine acidity strength; they reflect other physical properties that are not indicative of how the acid behaves in solution. Thus, the ability of HCl to dissociate fully makes it a strong acid rather than these other characteristics.