Which statement correctly describes how acid rain forms?

• A. Ozone is the main contributor to acid rain.
• B. Carbon dioxide dissolves in water to form carbonic acid, lowering rain pH.
• C. Methane directly lowers rainfall pH by dissolving.
• D. Sulfur dioxide and nitrogen oxides react with water vapor to form sulfuric and nitric acids, lowering precipitation pH.

Answer: (D)

Acid rain forms mainly because sulfur dioxide and nitrogen oxides released from burning fossil fuels react in the atmosphere with water vapor to produce sulfuric acid and nitric acid. When these acids dissolve in rain, they lower the pH, making rain more acidic. This happens because SO2 is oxidized and hydrates to form sulfuric acid (H2SO4), while NOx gases lead to nitric acid (HNO3) through similar reactions in the presence of water and oxygen. Natural rain does have some acidity due to carbon dioxide forming carbonic acid, but the strong acidity of acid rain comes from these sulfuric and nitric acids produced by human activities. Methane doesn’t directly form acids in rain, and ozone isn’t the main driver of acid rain.