## Chemistry: The Molecular Science (5th Edition)

A rain with pH equal to 6.7 is not considered "acid rain", because due to the presence of $CO_2$ in the atmosphere, the unpolluted rain has, naturally, a pH that is slightly acidic (About 5.6).
These are the reactions: $CO_2 + H_2O \lt -- \gt H_2CO_3 + H_2O \lt -- \gt H_3O^+ + HCO{_3}^-$ - Since we have a natural reaction that produces $H_3O^+$, this rain is not considered "acid rain".