In this chemistry demonstration, acid rain is simulated in a petri dish. Sodium sulfite (NaSO3) and hydrochloric acid (HCl) are mixed to create sulfur dioxide gas (SO2), which is a common byproduct of industrial processes. In the air, this gas mixes with rainwater (H2O) and then with oxygen (O2) to produce sulfuric acid (H2SO4). In the petri dish, this same reaction occurs and "acid rain" is produced. The pH change of each of the reactants and products is shown by a few drops of bromothymol blue--a common pH indicator. 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.