Normal rain's slight acidity is largely due to the presence of which gas?

Normal rain is slightly acidic primarily due to the presence of carbon dioxide in the atmosphere. When carbon dioxide dissolves in rainwater, it reacts with water to form carbonic acid, which contributes to the overall acidity of rain. This natural process gives rain its typical pH of around 5.6, making it slightly acidic rather than neutral.

Other gases, such as nitrogen and oxygen, do not significantly contribute to the acidity of rainwater. While sulfur dioxide can lead to more acidic rain, known as acid rain, in normal conditions, carbon dioxide is the main factor responsible for the slight acidity typically observed in regular rainfall.