Vapor Density is used to compare a vapor's density to what reference?

• A. Water
• B. Dry air
• C. Humid air
• D. Oxygen

Answer: (B)

Vapor density is the comparison of a vapor’s density to the density of dry air at the same temperature and pressure. Dry air is used as the reference because it provides a consistent standard that isn’t affected by the moisture content in the surrounding air. If the vapor density is greater than 1, the vapor is heavier than air and tends to collect near the ground; if it’s less than 1, it’s lighter and will rise. This relies on the idea that gas density at a given conditions is proportional to its molecular weight, so comparing to dry air (about 29 g/mol) lets you anticipate how the vapor behaves in a room.