What substance combines with leaking chlorine gas to form a white smoke?

The correct answer is ammonia vapor, as it reacts with chlorine gas to form a white smoke known as chlorine gas (Cl2) and ammonium chloride (NH4Cl). This reaction occurs because ammonia is a basic substance that can neutralize the acidic properties of chlorine gas. When these two gases combine, especially in the presence of moisture, they form visible white fumes due to the fine particles of ammonium chloride.

In contrast, other substances listed do not create the same reaction with chlorine gas. For instance, while carbon dioxide might dissolve in water to form a slightly acidic solution, it does not react to create a white smoke. Water vapor itself would not produce a smoke when mixed with chlorine; it may dissolve in chlorine but won’t showcase distinct visual indicators. Sulfur dioxide can react with some chlorine compounds, but it does not lead to the formation of a white smoke as ammonia does. Recognizing these chemical behaviors and reactions can be critical, especially in scenarios involving chemical spills or leaks.