Which substance is considered a strong acid?

Hydrochloric acid is considered a strong acid because it completely dissociates in aqueous solution. This means that when hydrochloric acid is dissolved in water, it breaks apart entirely into hydrogen ions (H⁺) and chloride ions (Cl⁻). This complete ionization leads to a high concentration of hydrogen ions in solution, significantly decreasing the pH and exhibiting the typical characteristics of strong acids, such as a strong sour taste and the ability to conduct electricity.

In contrast, acetic acid, carbonic acid, and phosphoric acid are classified as weak acids. These substances do not fully dissociate in solution, which results in a lower concentration of hydrogen ions compared to strong acids. Consequently, their pH levels reflect weaker acidic behavior, making them less effective in processes that require strong acidity, such as certain reactions in titrations or in catalysis. Therefore, hydrochloric acid stands out as the main representative of strong acids among the listed choices.