What is vapor density defined as?

Vapor density is defined as the ratio of the average molecular weight of a given volume of gas to that of air. This definition is crucial in fire investigations and safety assessments because it helps determine whether a gas will rise or sink in the atmosphere. Gases that are lighter than air (with a vapor density less than 1) will rise, while those that are heavier (with a vapor density greater than 1) will settle closer to the ground. Understanding vapor density is essential for evaluating potential hazards from flammable or toxic gases, especially in confined spaces.

In contrast, the other options address different physical concepts, such as heat reactions or pressure in confined spaces, which do not pertain directly to the definition of vapor density. These alternatives do not reflect the intrinsic relationship between a gas's molecular weight and air, which is central to understanding vapor density in the context of fire safety and investigation.