Acid rain is exactly what it suggests- rain that is acidic. The definition of "acid rain" is rain with a pH of below 5.6. Rain becomes acidic because of gases that dissolve in the rain. Approximately 70% of acid rain is a result of dissolved sulfur dioxide (SO2) which forms Sulfuric Acid. The remaining 30% or so comes from various Nitrogen Oxides (mostly NO2 and NO3 which has collectedly adopted the mane Nox). There is also a small percentage of hydrochloric acid that makes up hydrochloric acid as well. The equation is as follows: Sulfuric acid when it joins with hydrogen atoms in the air: In the air, the nitrogen ion becomes nitric or nitrous acid: NO2(g) + H2O(l) = HNO3(aq) +