Texas Commission on Environmental Quality (TCEQ). Class C Water License Exam

Which compounds are formed when chlorine gas is added to water?

1. Sodium hydroxide and hydrochloric acid

2. Hydrochloric acid and hypochlorous acid

3. Hydrogen peroxide and chlorine dioxide

4. Chlorinated organics and water

The correct answer is: Hydrochloric acid and hypochlorous acid

When chlorine gas is added to water, it reacts to form hydrochloric acid and hypochlorous acid. This reaction can be represented by the equation: Cl₂ + H₂O → HCl + HOCl In this process, chlorine gas (Cl₂) dissolves in water and undergoes hydrolysis, resulting in the formation of the strong acid hydrochloric acid (HCl) and the weaker acid hypochlorous acid (HOCl). Hypochlorous acid is particularly significant because it is a useful disinfectant commonly found in water treatment processes due to its ability to kill bacteria and other pathogens. The presence of hydrochloric acid contributes to the acidity of the solution but is not the sole reason for the disinfecting properties of chlorinated water. Other options do not accurately reflect the chemical changes that occur when chlorine gas is introduced to water. For example, while sodium hydroxide and hydrochloric acid is an interesting pair, this combination does not arise from the addition of chlorine gas to water. Hydrogen peroxide and chlorine dioxide are also unrelated products in this context, as they are typically generated through different chemical processes. Finally, while chlorinated organics can form in some circumstances when chlorine interacts with organic compounds, this does not describe