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

The pH scale is a measurement of the acidity or basicity of a solution. It is an essential concept in chemistry, allowing us to classify substances as acids, bases, or neutral. The scale runs from 0 to 14, with 7 as the neutral midpoint. Acids have a pH below 7, with lower pH values indicating stronger acidity. Bases, on the other hand, have a pH above 7, where higher values signify stronger basicity.

The scale is logarithmic, meaning each whole pH value below 7 is ten times more acidic than the next higher value. For example, a solution with a pH of 3 is ten times more acidic than one with a pH of 4. This logarithmic nature helps us easily compare the relative strength of acids and bases.

The pH of a solution is determined by the concentration of hydrogen ions, denoted as \( H^+ \) or sometimes as \( H_3O^+ \) for the hydronium ion. A higher concentration of hydrogen ions corresponds to a lower pH and therefore a more acidic solution. Water, considered neutral, has a pH of 7 because the concentrations of hydrogen ions and hydroxide ions are equal.

Carbon dioxide (CO₂) in the atmosphere plays a significant role in the natural acidity of rainwater. When CO₂ dissolves in water, it forms a weak acid known as carbonic acid \( H_2CO_3 \). The presence of carbonic acid in solution is a result of the following equilibrium reaction:\[ CO_2(g) + H_2O(l) \leftrightharpoons H_2CO_3(aq) \]Carbonic acid is considered a weak acid because it does not completely dissociate in water. Instead, it forms a balance between dissociated ions and undissociated acid. The association with water and the subsequent partial dissociation results in the production of hydrogen ions \( H^+ \) and bicarbonate ions \( HCO_3^- \). It's the release of these hydrogen ions that imparts the acidic characteristic to rainwater. This natural process makes rainwater more acidic than pure water, but still less acidic compared to rain influenced by pollutant gases, such as sulfur dioxide or nitrogen oxides, which create stronger acids upon dissolution.

The process of carbon dioxide reacting with water to form carbonic acid, which then dissociates into hydrogen ions and bicarbonate, is characterized by a dynamic equilibrium. In a chemical equilibrium, the rate at which the reactants form the products is equal to the rate at which the products reform the reactants. This can be represented as:\[ H_2CO_3(aq) \leftrightharpoons H^+(aq) + HCO_3^-(aq) \]At equilibrium, the concentration of each constituent doesn't change, despite the ongoing dynamic process. The extent to which carbonic acid dissociates is governed by its acid dissociation constant. Given that carbonic acid is a weak acid, the equilibrium is far to the left, meaning there are more undissociated carbonic acid molecules than dissociated ions. This is why the pH of pure rainwater remains around 5.6, which is acidic but not as low as one might expect if carbonic acid fully dissociated. The equilibrium can shift with changes in environmental conditions like temperature and pressure, but in the natural atmosphere, it remains relatively stable, ensuring that pure rainwater has a consistent, although slightly acidic, pH in the absence of additional pollutants.