Final answer:
Nitric acid is considered a strong acid because it completely dissociates in aqueous solution. The leveling effect in water masks the differences in strength among strong acids, which is why solvents less basic than water are used to compare their strengths.
Step-by-step explanation:
The confusion around nitric acid's classification as a strong acid arises from its pKa value, which is close to that of the hydronium ion. However, nitric acid (HNO3) is indeed classified as a strong acid because it dissociates completely in aqueous solution, producing nitrate ions (NO3-) and hydronium ions (H3O+). The pKa value suggests relative acidity in a different context and doesn't affect this classification.
The concept to understand here is the leveling effect. This is the phenomenon where strong acids all appear equally strong in water because they all donate protons to water to form hydronium ions, which is the strongest acid that can exist in water. This is why we only see H3O+ in solution for strong acids and cannot directly measure differences in their strengths in aqueous solution.
To observe the differences between strong acids, one would have to use a solvent less basic than water. For example, hydrogen iodide (HI) is completely dissociated in acetic acid, whereas nitric acid partially dissociates, demonstrating that HI is a stronger acid in this non-aqueous environment.