Carbon dioxide is often referred to as an "acidic gas" due to its ability to react with water to form carbonic acid (H2CO3). When carbon dioxide dissolves in water, it undergoes a chemical reaction known as hydration. This reaction results in the formation of carbonic acid, which can dissociate to release hydrogen ions (H+) into the solution, making it slightly acidic.
The reaction can be represented as follows:
CO2 + H2O → H2CO3
Carbonic acid is a weak acid, meaning it partially dissociates in water, releasing a small number of hydrogen ions. However, even this slight increase in hydrogen ion concentration is sufficient to lower the pH of the solution and make it acidic. The pH scale ranges from 0 to 14, with values below 7 indicating acidity.
In addition to its ability to form carbonic acid, carbon dioxide can also react directly with bases to form carbonates and bicarbonates, which can contribute to the alkalinity of a solution. This is particularly relevant in natural systems, such as the ocean, where carbon dioxide dissolution can lead to ocean acidification due to the formation of carbonic acid.
Overall, carbon dioxide is called an acidic gas because of its ability to form carbonic acid when dissolved in water, thereby increasing the hydrogen ion concentration and decreasing the pH of the solution.