* When NO₂ dissolves in water: It reacts with water to form nitric acid (HNO₃) and nitrous acid (HNO₂):

* 2 NO₂ + H₂O → HNO₃ + HNO₂

* Nitric acid (HNO₃): This is a strong acid, meaning it readily donates a proton (H⁺) in solution.

* Nitrous acid (HNO₂): This is a weak acid, but it still contributes to the acidic nature of the solution.

Therefore, the presence of these acids in the solution makes nitrogen dioxide acidic.