1. Normal Rain vs. Acid Rain: Normal, unpolluted rainwater is slightly acidic with a pH of approximately $5.6$.
This acidity is natural and occurs because carbon dioxide ($CO_2$) from the atmosphere dissolves in water to form weak carbonic acid ($H_2CO_3$):
$$CO_2(g) + H_2O(l) \rightleftharpoons H_2CO_3(aq)$$
2. Formation of Acid Rain: Acid rain occurs when fossil fuels are burned, releasing sulfur dioxide ($SO_2$) and nitrogen oxides ($NO_x$) into the atmosphere.
These gases react with water, oxygen, and other chemicals to form sulfuric acid ($H_2SO_4$) and nitric acid ($HNO_3$), which are much stronger than carbonic acid.
3. Typical pH Range: When these strong acids mix with precipitation, the pH of the rainwater drops below the natural threshold of $5.6$.
The generally accepted range for acid rain is between