Which of the following substances is acidic?

Nitrogen dioxide is considered acidic because it can react with water to form nitric acid (HNO₃) and nitrous acid (HNO₂). When nitrogen dioxide dissolves in water, it undergoes a reaction that leads to the liberation of protons (H⁺ ions), which characteristic of acidic substances. This ability to donate protons is a defining feature of acids according to the Brønsted-Lowry acid theory.

In contrast, the other substances listed do not exhibit this acidic behavior. Magnesium oxide is a basic oxide, which means it tends to react with acids rather than exhibiting acidic properties. Ferrous oxide, which is also a basic oxide formed from iron, similarly acts as a base. Calcium carbonate is a salt that can neutralize acids and does not donate protons in solutions, indicating that it is not acidic either. Therefore, among the listed choices, nitrogen dioxide stands out as the one that fits the definition of an acidic substance.