Relative density

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Relative density, also called specific gravity, is a dimensionless quantity defined as the ratio of the density (mass divided by volume) of a substance to the density of a given reference material. The relative density of solids and liquids is nearly always measured with respect to water at its densest (at 4 °C or 39.2 °F); for gases, the reference is air at room temperature (25 °C or 77.0 °F). The term "relative density" (abbreviated r.d. or RD) is preferred modern use, for example in ISO, IUPAC, and NIST, while the term "specific gravity" (abbreviated S.G. or SG) is gradually being abandoned. If a substance's relative density is less than 1 then it is less dense than the reference; if greater than 1 then it is denser than the reference. If the relative density is exactly 1 then the densities are equal; that is, equal volumes of the two substances have the same mass. If the reference material is water, then a substance with a relative density less than 1 will float in water. For example...

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