In a closed area, gases with a vapor density less than air will:

• A. Rise and not concentrate near the ceiling.
• B. Rise and concentrate near the ceiling.
• C. Settle and not concentrate at the floor.
• D. Settle and concentrate at the floor.

Answer: (B)

Gases that have a vapor density lighter than air are buoyant in the air, so they rise. In a closed space, with limited ventilation, this lighter gas will accumulate near the ceiling rather than at the floor. So the gas described will rise and concentrate up near the ceiling. If the gas were heavier than air, it would tend to settle toward the floor instead.