Final answer:
The primary difference between dilute and concentrated nitric acid is the concentration of HNO3 molecules, with dilute solutions having a lower concentration of these molecules than concentrated solutions.
Step-by-step explanation:
The primary difference between dilute nitric acid and concentrated nitric acid is the concentration of nitric acid molecules present in the solution. A dilute nitric acid solution has a lower concentration of nitric acid (HNO3) molecules compared to a concentrated nitric acid solution. It is not necessarily about pH, as both are strong acids and have a very low pH when dissolved in water, nor about the concentration of nitrate ions specifically, since the concentration of nitrate ions directly relates to the concentration of the acid itself. Additionally, both dilute and concentrated nitric acid can have a strong acidic effect on metals, but the exact nature of this reaction can vary depending on the concentration of the nitric acid.