Question

Why is rainwater naturally acidic, even in the absence of polluting gases such as \(\mathrm{SO}_{2}\) ?

Short answer

Rainwater is naturally acidic due to the presence of dissolved carbon dioxide (CO₂) in the atmosphere. When CO₂ dissolves in water, it forms carbonic acid (H₂CO₃) through the following reaction: \( CO_2(aq) + H_2O(l) \rightarrow H_2CO_3(aq) \). This carbonic acid then dissociates to a certain extent, forming hydrogen ions (H⁺) and bicarbonate ions (HCO₃⁻) as follows: \( H_2CO_3(aq) \rightleftharpoons H^+(aq) + HCO_3^-(aq) \). The presence of hydrogen ions causes acidity. Therefore, even in the absence of polluting gases, the pH of pure rainwater is around 5.6, making it slightly acidic due to this natural process.

Step-by-step solution

Dissolution of Carbon Dioxide in Water

CO₂ is a major constituent of the earth's atmosphere. When it comes into contact with water, it dissolves and forms a weak chemical bond with the water molecules. The reaction can be represented as follows: \( CO_2(g) \rightarrow CO_2(aq) \)

Formation of Carbonic Acid

The dissolved CO₂ in water undergoes a chemical reaction to form carbonic acid (H₂CO₃). This reaction can be represented as: \( CO_2(aq) + H_2O(l) \rightarrow H_2CO_3(aq) \)

Dissociation of Carbonic Acid

Carbonic acid is a weak acid, meaning it does not fully dissociate in water. However, it does dissociate to a certain extent, forming hydrogen ions (H⁺) and bicarbonate ions (HCO₃⁻). The dissociation reaction can be represented as: \( H_2CO_3(aq) \rightleftharpoons H^+(aq) + HCO_3^-(aq) \)

pH and Acidity of Rainwater

The dissociation of carbonic acid in rainwater forms hydrogen ions (H⁺) which cause acidity. The pH of a solution is a measure of the concentration of hydrogen ions in it. The more hydrogen ions present, the lower the pH and the more acidic the solution. The pH scale ranges from 0 to 14, with 7 being neutral. Any solution with a pH below 7 is considered acidic, while a pH above 7 indicates a solution is basic or alkaline. Due to the presence of dissolved CO₂ and the subsequent formation of carbonic acid, the pH of pure rainwater is around 5.6, making it slightly acidic. Even in the absence of pollution, rainwater's acidity will result from this natural process.