The pH scale is a simple numerical scale from 1 to 14. If you take a piece of Litmus paper and dip it in a solution (we’ll go with rain water if you like) and it comes out neutral, the pH value would be 7, right in the middle. It would be neither acidic, nor alkaline, and that’s how rainwater is supposed to be…pure. If pH is below 7, the water is acidic and if above 7 it is alkaline. Thus, values of 6 and 8 would be slightly acidic and slightly alkaline, respectively. A pH value of 1 or 14 would be an extremely dangerous substance (either very, very acidic or very very alkaline).
The pH of modern rainwater is between 5.5 and 6.0 because of carbon dioxide that’s dissolved in the atmosphere. This CO2 reacts with the H2O (water vapor) in the atmosphere for the following reaction:
CO2 + H2O —> H2CO3 (carbonic acid)
then that dissociates:
H2CO3 —> H+ + HCO3- (bicarbonate ion)
the bicarbonate ion can then dissociate:
HCO3- —> H+ + CO3- (carbonate).
All of these reactions are reversible as well and is what makes bicarbonate a really good buffer. Bicarbonate is the same buffer used in human blood.
Ph Of Rainwater
normally rain water has a pH of 5.6 due to the presence of H+ ions formed by the reaction of rain water with water with carbon dioxide present in the atmosphere. When the pH of the rain water drops below 5.6, its is called called acid rain.
Unpolluted rain water has a pH of around 5.2, which is a result of the presence of carbonic acid derived from CO2 in the atmosphere. If there are other pollutants such of SO2 and SO3 the pH can be much more acidic because these will for sulfurous and sulfuric acid in the rain.
simpliest numerical scale