Like it says on the tin. Several sites say that the pH of rainfall is somewhere between 5 and 5.5, and a few seem to suggest that it averages around 5.7. But why is the pH of rain much lower than that if pure water, which is 7?
This is because of the various oxides present in the air. Oxides of sulphur, nitrogen and carbon are always present in the air. They react with the water during rainfall to form their respective acids as shown below :
SO3+ H2O -> H2SO4
2NO2 + H2O -> HNO3 + HNO2
SO2 + H20 -> H2SO3
CO2 + H2O -> H2CO3
Hence, due to the presence of these acids the pH of rain is less than 7. When the levels of these oxides in the air increase due to excessive pollution it leads to further decreasing of the pH of rain which causes to acid rain.