The pH of natural, unpolluted rainwater is typically around 5.6.Rainwater is naturally slightly acidic because carbon dioxide (CO2) from the atmosphere dissolves in it, forming weak carbonic acid (H2CO3).Acid rain, caused by pollutants like sulfur dioxide (SO2) and nitrogen oxides (NOx), has a pH lower than 5.6. Pure water has a neutral pH of 7.0, but rainwater is never pure due to dissolved atmospheric gases.