Learners add acid rain (nitric acid) to two cups that represent lakes. One cup contains limestone gravel and the other contains granite gravel. Learners also add an indicator to each solution that shows any change in the acidity of the "lakes." The indicator shows that the limestone lake returns to neutral while the gravel lake becomes acidic. This activity is part of the Environmental Chemistry Unit in OMSI's Chemistry Lab. Materials estimates are for 100 uses. For safety reasons, this activity works best as a demonstration for younger audiences.