Specific gravity, also known as relative density, is a dimensionless quantity that represents the ratio of the density of a substance to the density of a reference substance, typically water at 4°C for solids and liquids. It is mathematically expressed as:
SG=ρsubstanceρwaterSG=\frac{\rho_{\text{substance}}}{\rho_{\text{water}}}SG=ρwaterρsubstance
where ρsubstance\rho_{\text{substance}}ρsubstance is the density of the substance and ρwater\rho_{\text{water}}ρwater is the density of water at its densest state
. Because it is a ratio of two densities with the same units, specific gravity has no units. It indicates whether a substance will float or sink in water: if the specific gravity is less than 1, the substance will float; if it is greater than 1, it will sink
. For gases, specific gravity is often measured relative to air instead of water. For example, methane has a specific gravity of about 0.55 relative to air, meaning it is lighter and will rise in the atmosphere
. In summary, specific gravity is a measure of how dense a substance is compared to water (or air for gases), used widely in science and industry to identify substances and predict buoyancy
