Which of the following gases is heavier than air, making it hazardous at ground level?

Propane is heavier than air, and this characteristic is crucial for understanding its behavior in the environment. Gases that are heavier than air tend to accumulate in low-lying areas, which can pose serious hazards as they may displace oxygen and create an asphyxiation risk. This is particularly relevant in scenarios like a gas leak, as propane that escapes can settle in basements or other enclosed spaces where ventilation is poor.

In contrast, the other gases listed—oxygen, nitrogen, and helium—are lighter than air. Oxygen, while essential for combustion and respiration, will rise and mix with the atmosphere if released. Nitrogen, comprising a significant part of the air we breathe, will also dissipate upwards rather than collect at ground level. Helium is much lighter than air, causing it to rise quickly. Recognizing the properties of these gases, especially in terms of density relative to air, is vital for safety and risk assessment in various environments.