Here's why:

* Acid rain is formed when sulfur dioxide (SO2) and nitrogen oxides (NOx) are released into the atmosphere. These gases react with water, oxygen, and other chemicals to form sulfuric acid (H2SO4) and nitric acid (HNO3).

* These acids then dissolve in rainwater, making it more acidic.

* The resulting acid rain is a mixture of water, sulfuric acid, nitric acid, and other compounds.

Key takeaway: Acid rain is a mixture because it contains multiple substances that are not chemically bonded together.