Chlorine gas is known to be:

Chlorine gas is heavier than air, which is an important characteristic to understand in water distribution and treatment processes. As chlorine is released, it tends to settle toward the ground rather than dissipating into the atmosphere. This property is critical for safety protocols in environments where chlorine is used, as any accumulation of chlorine gas can create hazardous conditions, particularly in low-lying areas or confined spaces.

In contrast, other characteristics of chlorine include that it is greenish-yellow, has a distinct, strong odor, and is toxic, which means that the options suggesting it is colorless and odorless or non-toxic are not accurate. Understanding the behavior of chlorine gas emphasizes the need for proper ventilation and safety measures when handling it in water treatment facilities.