What term describes the weight of a given volume of pure vapor or gas compared to the weight of an equal volume of dry air at the same temperature?

• A. Vapor Density
• B. Boiling Point
• C. Solubility
• D. Viscosity

Answer: (A)

Vapor density describes how heavy a given volume of pure vapor or gas is compared with the same volume of dry air at the same temperature. It’s essentially a comparison of weights: if the vapor density is greater than 1, the gas is heavier than air and will tend to pool in low areas and stay near the ground; if it’s less than 1, the gas is lighter than air and will rise and disperse. This concept helps responders predict where leaks might collect and how the gas will spread, which is crucial for safe ventilation, evacuation, and ignition control. Boiling point, solubility, and viscosity involve different properties (temperature at which a liquid boils, how well it dissolves, and resistance to flow) and don’t describe the comparative weight of vapor to air.