Which gas is lighter than air in plumbing?

• A. Propane
• B. Methane
• C. Natural Gas
• D. Butane

Answer: (C)

Gas that is lighter than air has a lower density than the surrounding air, so it rises when released. In residential plumbing, the gas delivered to homes is natural gas, which is primarily methane. Methane has a specific gravity well below 1, meaning it will rise and mix with the air rather than pool on the floor. Because natural gas shares this same light characteristic, it’s the gas most associated with leaks rising toward the ceiling in plumbing contexts. Propane and butane are heavier than air, so they would behave differently by tending to collect low. While methane itself is lighter than air, the plumbing context is tied to the gas supplied to homes, making natural gas the best choice.