Nitrogen Dioxide Forms Acid Rain Eroding Marble

Steps:

• Identify emissions from internal combustion engines: CO, NOx (including NO and NO2), and unburnt hydrocarbons.
• Link emissions to environmental effects: NOx contributes to acid rain formation.
• Determine impact on marble statues: Acid rain reacts with calcium carbonate in marble, causing erosion.
• Match to choices: Nitrogen dioxide (NO2) is the key NOx species responsible.

Why C is correct:

• Nitrogen dioxide (NO2) reacts with atmospheric water to form nitric acid (HNO3 via 3NO2 + H2O → 2HNO3 + NO), which dissolves calcium carbonate in marble (CaCO3 + 2HNO3 → Ca(NO3)2 + H2O + CO2).

Why the others are wrong:

• A. Carbon monoxide (CO) is a toxic gas but does not form acids that erode marble.
• B. Nitrogen oxide (NO) is a precursor to NO2 but does not directly form nitric acid.
• D. Unburnt hydrocarbons contribute to smog and ozone but not to acid rain affecting marble.

Final answer: C