What defines the specific gravity of a gas?

The specific gravity of a gas is defined as the weight of that gas compared to an equal volume of air at standard conditions. This means that specific gravity is a measure of how dense the gas is relative to air, which allows for comparisons of different gases in terms of their buoyancy and behavior when mixed with air.

When discussing specific gravity, it is crucial to understand that air is often used as the reference because it is the medium in which the gas will typically interact. If a gas has a specific gravity less than 1, it will rise in air, while a gas with a specific gravity greater than 1 will sink. This concept is essential for applications involving gas systems, such as predicting how a gas will behave in an environment and ensuring safe handling and use.

The other definitions provided do not accurately describe specific gravity. Comparing the weight of a gas to oxygen does not establish the relationship with air, which is standard for specific gravity measurements. The volume of gas at a given temperature refers to concepts from gas laws rather than specific gravity, and atmospheric pressure is a separate factor influencing gas behavior but not a direct definition of specific gravity.