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 carbon dioxide (CO₂) in the atmosphere. When carbon dioxide dissolves in water, it forms carbonic acid (H₂CO₃), which ionizes to form bicarbonate ion (HCO₃⁻) and a hydrogen ion (H⁺). Bicarbonate ion can further ionize to form carbonate ion (CO₃²⁻) and another hydrogen ion (H⁺). The presence of H⁺ ions makes the rainwater acidic, even in the absence of polluting gases like sulfur dioxide (SO₂).

Step-by-step solution

Formation of Carbonic Acid

Carbon dioxide (CO₂) is present in the atmosphere naturally. When CO₂ dissolves in water (H₂O), it forms carbonic acid (H₂CO₃). The chemical reaction can be represented as follows: \[ CO_{2} + H_{2}O \rightarrow H_{2}CO_{3} \]

Ionization of Carbonic Acid

Carbonic acid is a weak acid that can undergo ionization and release hydrogen ions (H⁺), making the solution acidic. In water, carbonic acid ionizes to form bicarbonate ion (HCO₃⁻) and a hydrogen ion (H⁺). \[ H_{2}CO_{3} \rightleftharpoons HCO_{3}^{-} + H^{+} \]

Further Ionization of Bicarbonate Ion

Bicarbonate ion can undergo further ionization to form carbonate ion (CO₃²⁻) and a hydrogen ion (H⁺). \[ HCO_{3}^{-} \rightleftharpoons CO_{3}^{2-} + H^{+} \] Since H⁺ ions are produced through this process, the presence of carbon dioxide in the atmosphere leads to the formation of acidic substances in rainwater, making it naturally acidic even in the absence of other polluting gases like sulfur dioxide (SO₂).