Which of the following is NOT considered a strong acid?

The answer is based on the classification of acids in terms of their strength, measured by their ability to dissociate in water and donate protons (H+ ions).

H2SO4 (sulfuric acid), HClO4 (perchloric acid), and HNO3 (nitric acid) are all classified as strong acids. They fully dissociate in aqueous solution, releasing their protons completely, which contributes to their high conductivity and low pH when dissolved in water.

On the other hand, H2SO3 (sulfurous acid) does not fully dissociate in water, making it a weak acid. While it can release some protons, it does not do so to the extent that the strong acids do. Therefore, it is not considered a strong acid, confirming that it is the correct answer to the question regarding which substance is not classified as a strong acid. Understanding the distinction between strong and weak acids is crucial in the study of chemistry, as it informs chemical behavior, reactivity, and applications in various contexts.