Which of the following is NOT a strong acid?

The reasoning behind identifying H2SO3 as not being a strong acid lies in its properties and behavior in aqueous solution. Strong acids are defined by their ability to completely dissociate in water, meaning they release protons (H+) readily and significantly enhance the concentration of hydrogen ions in the solution.

Nitric acid (HNO3), perchloric acid (HClO4), and hydrochloric acid (HCl) are all classified as strong acids because they dissociate fully when dissolved in water. This means that in significant concentrations, these acids contribute a high concentration of H+ ions to the solution, resulting in a lower pH and a very acidic environment.

In contrast, sulfurous acid (H2SO3) is a weak acid. It does not fully dissociate in solution, meaning only a fraction of H2SO3 molecules release protons. This partial dissociation results in a relatively lower concentration of H+ ions compared to strong acids, leading to a higher pH and a less acidic environment.

Therefore, the distinguishing factor that identifies H2SO3 as not a strong acid is its incomplete dissociation in solution, in direct contrast to the behavior exhibited by the strong acids listed in the other options.