Acid rain is precipitation with a low pH caused by

• A. Precipitation caused by carbon sequestration.
• B. A form of clean rain.
• C. Precipitation with high pH.
• D. Precipitation with a low pH caused mostly by sulfur dioxide and nitrogen oxides.

Answer: (D)

Acid rain forms because sulfur dioxide and nitrogen oxides released from burning fossil fuels react in the atmosphere to form sulfuric and nitric acids. These acids dissolve in rain droplets, lowering the rainfall’s pH so it becomes acidic rather than neutral or basic. This is driven by the chemical transformation of those gases into airborne acids, not by processes like carbon sequestration or by rain that is simply clean or high in pH. The essential idea is that the low pH of the precipitation is caused mainly by sulfur dioxide and nitrogen oxides, which makes it acidic and capable of causing environmental damage.