What causes the pH to decrease when chlorine gas is added to water?

The correct answer is that two acids form when chlorine gas is added to water. When chlorine gas is introduced to water, it reacts with it to form hydrochloric acid and hypochlorous acid. This reaction results in an increase in hydrogen ions (H+) in the solution, which decreases the pH, making the water more acidic. The formation of these acids is significant because it impacts the overall balance of the water chemistry, affecting not only pH but also other properties such as disinfection efficiency and corrosivity.

In contrast, the other options do not accurately describe the chemical reactions occurring when chlorine is added to water. The formation of caustic substances or caustic soda does not directly occur with the addition of chlorine gas; in fact, it's the opposite reaction that leads to an acidic environment. Similarly, the suggestion of forming two base materials does not reflect the reality of how chlorine interacts with water, as chlorine does not generate bases in this scenario. Therefore, understanding the nature of the acids produced helps clarify why there is a decrease in pH when chlorine gas is added to water.