When discussing "Relative Density," the density is compared to what substance?

Relative density, often referred to as specific gravity, is a measure that compares the density of a substance to the density of a reference substance. In most contexts, especially in science and engineering, the reference substance is water. This means that when determining the relative density of a solid or liquid, its density is compared to the density of water, which is considered to have a specific gravity of 1 at standard temperature and pressure.

Using water as the reference is practical because it is abundant and has well-known properties. By comparing other substances to water, one can easily understand if they are denser or less dense than water, which aids in various applications, including buoyancy calculations and material sciences.

While air, oxygen, and mercury can also be compared in discussions of density, they are not the standard reference for relative density. Air is less dense than most liquids, making it an unsuitable reference for liquids, and while mercury is denser than water and may be used in specific scenarios, it is not the conventional reference for reporting relative density. Thus, water is the appropriate substance for this comparison.