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

The term that refers to the weight of a gas compared to air is vapor density. Vapor density is defined as the ratio of the mass of a certain volume of gas to the mass of an equal volume of air, typically measured under the same temperature and pressure conditions. This comparison is crucial in various applications, especially in firefighting and hazardous materials response, as it helps determine whether a gas will rise or settle in the atmosphere.

Understanding vapor density is important because it can influence how the gas behaves once released, which can affect firefighting strategy and safety measures. For instance, gases with a vapor density greater than one tend to settle near the ground, while those with a vapor density less than one rise.

The other options, while related to gases, refer to different concepts: gas weight is not a standard term used in this context, air density refers to the mass of air, and vapor pressure denotes the pressure exerted by a vapor in equilibrium with its liquid or solid form. These distinctions highlight why vapor density is the appropriate choice for comparing the weight of a gas to that of air.