Massachusetts Wastewater Practice Exam

Carbon dioxide is classified as an acid, specifically when it reacts with water to form carbonic acid (H2CO3). This is a fundamental concept in chemistry, where certain substances can act as acids by donating protons (H+) or creating an acidic environment in solution. In the case of carbon dioxide, its ability to lower the pH of water by forming carbonic acid exemplifies its acidic characteristics.

The other chemicals listed do not demonstrate acidic properties in the same way. Potassium hydroxide is a strong base, sodium sulfite is a salt derived from a weak acid and a strong base, and potassium permanganate is usually considered a neutral compound in aqueous solutions. Therefore, when evaluating the nature of these chemicals, carbon dioxide stands out clearly as the one that exhibits acidic behavior through its interaction with water.