Carbonic acid refers to the natural existent acid in our atmosphere which is produced when water reacts with water vapor and it makes rain naturally acid. As a result, rainwater is always acidic. On the other hand, acid rain occurs when precipitation contains a higher than normal acidity and a pH lower than 5.0.
In this case, due to the increasing concentration of carbon dioxide in the atmosphere, when it rains, the following chemical reaction occurs between water and carbon dioxide:
Whose product is known as carbonic acid and it is the natural source of acidity in acid rain.
