______ is lighter than air, which means it will rise and accumulate in high areas and near ceilings.

• A. Propane
• B. Natural gas
• C. Acetylene
• D. Propylene

Answer: (C)

Gases that are lighter than air will rise and collect up toward the ceiling. This happens because their density is lower than that of the surrounding air, so when they escape they don’t mix evenly but tend to drift upward.

Acetylene has a molecular weight of about 26 g/mol, while air is roughly 29 g/mol. That makes acetylene lighter than air, so when it’s released it will rise and tend to accumulate near ceilings or in high corners. This is why leaks of lighter-than-air gases are often detected with ceiling-mounted detectors and why ventilation strategies in welding or cutting work focus on exchanging air at upper points.

In contrast, heavier-than-air gases would stay near the floor, which is why different safety practices apply depending on whether a gas tends to rise or stay low. The key idea here is the relative density to air driving where the gas collects.