In the Bronsted-Lowry acid-base theory, an acid is defined as a substance that donates a proton (H+ ion), and a base is a substance that accepts a proton. According to this theory, carbon dioxide (CO2) can be considered an acid because it can donate protons under certain conditions.
When dissolved in water, CO2 reacts with water to form carbonic acid (H2CO3) through the following equilibrium reaction:
CO2 + H2O ⇌ H2CO3
In this reaction, CO2 acts as an acid by donating a proton to water, forming the H2CO3 species. Carbonic acid, in turn, can further dissociate to produce hydrogen ions (H+) and bicarbonate ions (HCO3-):
H2CO3 ⇌ H+ + HCO3-
The presence of H+ ions indicates the acidic nature of carbon dioxide. These hydrogen ions can participate in acidic reactions, making CO2 behave as an acid according to the Bronsted-Lowry theory.
It is worth noting that the acidity of CO2 in aqueous solution is relatively weak compared to strong mineral acids like sulfuric acid (H2SO4) or hydrochloric acid (HCl). Carbon dioxide is considered a weak acid due to the relatively low concentration of hydrogen ions produced when dissolved in water. However, in the context of acid-base theories like Bronsted-Lowry, even weak acids such as CO2 can be classified as acids based on their ability to donate protons.