Acid rain is classified as a homogeneous mixture. This means that, despite the fact that it contains a mixture of different substances, the components are mixed uniformly at a molecular level.
The main components of acid rain are water, sulfuric acid, and nitric acid, which are created when sulfur dioxide and nitrogen oxides react with water vapor in the atmosphere. When these acids dissolve in raindrops, they create a consistent solution, meaning that, when you collect a sample of acid rain, it will have the same composition throughout.
In contrast, a heterogeneous mixture is one where the components are not uniformly distributed, and you can often see the different substances. Since acid rain does not exhibit these characteristics and its components are fully dissolved and evenly mixed, it qualifies as a homogeneous mixture.