Specific Gravity

Specific Gravity. Specific gravity is the quotient obtained by dividing the mass of a substance in a certain volume by the mass of water in the same volume. The numerical value of specific gravity is the same as that of density (see), which represents the mass, expressed in grams, contained in 1 cm of the given substance. Specific gravity is one of the most important physical-chemical characteristics of a substance, and in determining the purity of a substance its specific gravity is determined first. Specific gravity is a function of temperature, and with increasing temperature specific gravity usually decreases. There are certain substances that possess an anomalous course of specific gravity. In this respect water is of particular interest, possessing the greatest specific gravity at 3.99°, which however cannot be considered a constant value in view of the possible presence of heavy water, which has a point of greatest density at a higher t° than ordinary water. For determining the specific gravity of solids, the method of hydrostatic weighing is used. To one end A of the balance beam DAB (see figure) a body C, the specific gravity of which is to be determined, is suspended. The pressure of the liquid on the immersed body C, directed from below upwards, is equal, according to Archimedes' law, to the weight of the displaced liquid in the volume of the body. By placing weights on the scale pan R until the beam is again brought into equilibrium, the weight of the displaced liquid m is determined. The specific gravity g is equal to the ratio of the masses of the body and water in the volume of the body, or of the weight of the body and the weight of water in the volume of the body: g = m/M. For other methods of measuring specific gravity — see Hydrometer, Pycnometer.