The ratio of the density of a substance to the density of a reference substance; it is an abstract number that is unrelated to any units. For solids and liquids, specific gravity is numerically equal to density, but for gases it is not, because of the difference between the densities of the reference substances, which are usually water (1 g/cc) for solids and liquids and air (0.00129 g/cc, or 1.29 g/L at 0C and 760 mm Hg) for gases. The specific gravity of a gas is the ratio of its density to that of air; since the specific gravity of air = 1.0 (1.29/1.29), this is usually stated to indicate the comparison with the gas under consideration. For example, the density of hydrogen is 0.089 g/L but its specific gravity is 0.069 (i.e., 0.089/1.29). The specific gravity of solids and liquids is the ratio of their density to that of water at 4C, taken as 1.0, as 1 cc of water weighs 1 gram. Thus, a solid or liquid with a density of 1.5 g/cc has a specific gravity of 1.5/1 or 1.5.Since weights of liquids and gases vary with temperature, it is necessary to specify both temperatures involved, except for rough or approximate values. Thus, the specific gravity of alcohol should be given as 0.7893 at 20/4C, the first temperature referring to the alcohol and the latter to the water. At 15.56C the specific gravity of alcohol is 0.816.See Density; API Gravity; Baumé.
