The definition of acid rain is rain containing acids that form in the atmosphere and combine with water, regardless of the cause. Pollution, volcanic eruptions, power-generating plants, forest fires, and even lightning bolts are causes of acid rain. Acidic rain has been measured in several locations in the United States, Canada, and Scandinavia. The level of acidity in the rain varies across the United States and is higher in the east and lower in the west.
             Unpolluted rain or snow always is slightly acidic, having a Ph (an indicator of a solution's acidic or basic properties) value of 5.6. This is due to the carbon dioxide in the air that dissolves in the precipitation in the atmosphere to produce carbonic acid, a weak acid, such as what is found in carbonated drinks. If the Ph falls below 5.6, the rain or snow is considered acidic.
             This is how acid rain is formed. First, Sulfur dioxide, nitrous oxide, and nitrous dioxide are emitted into the air from natural and artificial sources. Next, these oxides are transported into the atmosphere. Then, there is a series of reactions between these oxides, moisture, and other chemicals in the atmosphere that form sulfuric acid, H2SO4, nitric acid, HNO3, and others. Finally, in precipitation, the substances being carried down as acid rain.
             Today there are many uncertainties associated with the issue of acid rain. Some of the significant gaps in our understanding of acid rain include the following. First is the question of the natural Ph level in the precipitation without human activity or interference. Second is the evidence of the change of the acidy over time. The acknowledgment of acid rain started in the 1800s; therefore, levels before this date are unknown, and the onset of this phenomenon is unclear. The third is the controlling factors in the production of sulfuric and nitric acids. Fourth is the exact mechanism for oxidizing sulfur dioxide and nitrogen oxides. Lastly, the ...