Why does rain water normally have a pH of about 5.6 ? When does it become acid rain ?

Explanation

Rain water has normally pH of 5.6. It is due to the formation of H" ion from the reaction of rain water with carbon dioxide present in the atmosphere. The reaction occurs as follows:
`H_2 O (l) + CO_2 (g) to H_2 CO_3 hArr 2H^+ (aq) + CO_3^(2-) (aq)`
Carbonic acid `(H_2 CO_3)` thus formed gives water of pH value nearly 5.6. When the pH of rain falls below 5.6. it becomes acid rain.