The pH of rainwater is typically slightly acidic, meaning it is less than 7. This is due to the presence of dissolved carbon dioxide (CO2) in the atmosphere.

Here's why:

* CO2 dissolves in water: When carbon dioxide from the atmosphere dissolves in water, it forms carbonic acid (H2CO3).

* Carbonic acid dissociates: Carbonic acid then dissociates into hydrogen ions (H+) and bicarbonate ions (HCO3-).

* Hydrogen ions increase acidity: The presence of hydrogen ions in the rainwater increases its acidity, resulting in a pH lower than 7.

The chemical reactions are as follows:

CO2(g) + H2O(l) ⇌ H2CO3(aq)

H2CO3(aq) ⇌ H+(aq) + HCO3-(aq)

Therefore, the pH of rainwater is naturally slightly acidic due to the dissolved carbon dioxide.

However, it's important to note that:

* Acid rain: Rainwater can become significantly more acidic (pH below 5.6) due to air pollution from human activities, such as burning fossil fuels. This is because pollutants like sulfur dioxide (SO2) and nitrogen oxides (NOx) react with water to form sulfuric acid (H2SO4) and nitric acid (HNO3), which further lower the pH.

* Natural variations: The pH of rainwater can vary depending on factors like location, temperature, and the presence of other atmospheric gases.

Overall, while rainwater naturally has a slightly acidic pH due to dissolved CO2, human activities can significantly contribute to acid rain, leading to serious environmental consequences.