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Rainwater is naturally acidic due to the presence of several gases found in the atmosphere, which dissolve in water and form weak acids. One of these is carbon dioxide (CO2), which is a natural constituent of Earth's atmosphere and dissolves in the water to form carbonic acid (H2CO3). Even when there are no polluting gases like sulfur dioxide (SO2) or nitrogen oxides (NOx), rainwater still has some degree of acidity due to the reaction with carbon dioxide.

Carbon dioxide is a greenhouse gas that is released into the atmosphere through various natural processes such as volcanic eruptions, respiration of living organisms, and decay of organic matter. When it comes in contact with water in the atmosphere, be it in clouds or water droplets, it dissolves and forms a weak solution of carbonic acid (H2CO3) according to the following reaction: \[CO_2(g) + H_2O(l) \rightleftharpoons H_2CO_3(aq)\]

Carbonic acid (H2CO3) formed in the reaction above is a weak acid, meaning it only partially ionizes in water. The ionization process involves donating a hydrogen ion (H+) to the water, forming two ions: bicarbonate ion (HCO3-) and hydronium ion (H3O+). \[H_2CO_3(aq) + H_2O(l) \rightleftharpoons HCO_3^-(aq) + H_3O^+(aq)\] The presence of hydronium ions (H3O+) in the solution is responsible for the acidic nature of rainwater.

The pH scale is used to measure the acidity or alkalinity of a solution. It ranges from 0 to 14, with 7 being neutral. Solutions with a pH less than 7 are considered acidic, while those with a pH greater than 7 are considered alkaline. In a pure water solution without any dissolved gases, the concentration of hydronium and hydroxide ions is equal, resulting in a pH value of 7. However, when carbon dioxide dissolves in water and forms carbonic acid, the concentration of hydronium ions increases, making the resulting solution acidic with a pH value below 7. The acidity of natural rainwater usually has a pH value of around 5.6, which is slightly acidic. In areas with higher levels of pollution, such as those with the presence of sulfur dioxide (SO2) and nitrogen oxides (NOx) gases, the acidity of rainwater can increase, leading to the formation of acid rain, which can have negative effects on plants, soil, and aquatic habitats.