What is the definition of specific gravity?

Specific gravity is defined as the ratio of the weight of a substance to the weight of an equal volume of a reference substance, typically water for liquids and air for gases. For gases, specific gravity measures how heavy a gas is when compared to the same volume of air at the same temperature and pressure. This measurement is crucial in plumbing and other fields to assess the buoyancy and behavior of gases as they relate to air.

In the context of gas, knowing specific gravity helps in understanding how a gas will behave in a mixture or in relation to other gases. Gases with a specific gravity less than 1 are lighter than air and will rise, whereas those with a specific gravity greater than 1 are heavier and will tend to sink. This property is essential in designing ventilation systems, gas lines, and understanding gas leaks.

The other choices do not accurately describe specific gravity; for example, the definition that refers to the weight of a substance compared to its liquid form is too vague, and the one comparing the density of a liquid to a gas mixes two different states without addressing the specific concept of gravity. The option discussing the pressure of gas compared to atmospheric pressure shifts the focus away from the concept of specific gravity entirely, relating instead to barometric measurements.