What does vapor density measure?

Answer: B — The weight of a vapor compared to air

Vapor density measures the weight of a vapor in comparison to the weight of air. It provides an important indication of how a vapor will behave in the atmosphere. If a vapor has a vapor density greater than 1, it is heavier than air and will tend to sink to lower areas, which can create safety hazards in confined spaces. Conversely, if the vapor density is less than 1, the vapor will rise. Understanding vapor density is critical for HAZMAT technicians when assessing potential risks and planning for safe responses during hazmat incidents. The other choices address unrelated concepts, with one focusing on different states of matter (liquid and solid), which do not accurately relate to the measurement of vapor density specifically.