* Carbon Dioxide Absorption: Rainwater absorbs carbon dioxide from the atmosphere as it falls.
* Carbonic Acid Formation: When carbon dioxide dissolves in water, it forms carbonic acid (H2CO3).
* Hydrogen Ions: Carbonic acid then partially dissociates, releasing hydrogen ions (H+) into the water.
* pH Change: The presence of these hydrogen ions makes the rainwater slightly acidic.
The Chemical Reaction:
CO2 (g) + H2O (l) ⇌ H2CO3 (aq) ⇌ H+ (aq) + HCO3- (aq)
Normal Rainwater pH:
The pH of normal rainwater is typically around 5.6. This is slightly acidic compared to pure water, which has a pH of 7.
Important Note: This natural acidity is different from acid rain, which is caused by human-produced pollutants like sulfur dioxide and nitrogen oxides. These pollutants react with water in the atmosphere to form stronger acids like sulfuric acid and nitric acid, making the rainwater much more acidic.
