Which term refers to the weight of a gas compared to air?

Which term refers to the weight of a gas compared to air?

• A. Pressure
• B. Vapor density
• C. Molecular weight
• D. Specific gravity

Answer: (B)

Vapor density is the weight of a gas relative to air at the same temperature and pressure. This measure directly tells you whether a gas will tend to stay low or rise: if the vapor density is greater than 1, the gas is heavier than air and will accumulate near the floor; if it’s less than 1, it’s lighter than air and will rise toward the ceiling. This concept is crucial for understanding how leaks behave, how to ventilate spaces, and where to place detectors. For example, propane has a vapor density around 1.5, so it tends to stay low, while methane is about 0.6, so it will rise. The other terms describe different properties: pressure is about force per area, molecular weight is the mass of a single molecule, and specific gravity compares density to water for liquids (with gases using vapor density to express their relation to air).