Which gas is described as colorless, odorless, and slightly lighter than air?

• A. Carbon dioxide (CO2)
• B. Hydrogen sulfide (H2S)
• C. Sulfur dioxide (SO2)
• D. Carbon monoxide (CO)

Answer: (D)

Understanding gas properties helps you identify them by what you can’t see or smell, and by how they behave in air. Carbon monoxide is the gas that is colorless and odorless, which means you wouldn’t detect it by sight or smell. It is also slightly lighter than air, with a molar mass a bit less than that of air, so it can mix with and spread through a space rather than settle in a low layer. This combination—no color, no odor, and being only a touch lighter than air—makes CO particularly dangerous in enclosed areas, since it can accumulate without warning and without notice.

In contrast, carbon dioxide is also colorless and odorless but heavier than air, so it tends to collect in low areas. Hydrogen sulfide has a distinct rotten-egg odor, so it isn’t odorless. Sulfur dioxide has a sharp, irritating odor and is heavier than air, so it behaves differently in the environment.