Specific Gravity

SPECIFIC GRAVITY. Specific gravity is the quotient obtained by dividing the mass of a substance in a given volume by the mass of water in the same volume. Specific gravity is numerically expressed by the same number that expresses density (see), which represents the mass expressed in grams contained in 1 cm³ of a given substance. Specific gravity is one of the most important physical-chemical characteristics of a substance, and when determining the purity of a substance, its specific gravity is determined first of all. Specific gravity is a function of temperature, and with an increase in temperature, specific gravity usually decreases. There are some substances that possess an anomalous course of specific gravity. In this regard, water is of particular interest, possessing the highest specific gravity at 3.99°, which, however, cannot be considered a constant value in view of the possible admixture of heavy water, which has a point of maximum density at a higher temperature than ordinary water. To determine the specific gravity of solid bodies, the method of hydrostatic weighing is used. A body C is suspended from one end A of the balance beam DAB (see figure), the specific gravity of which one wishes to determine.

with water. The pressure of the liquid on the immersed body C, directed from bottom to top, is equal, according to Archimedes' principle, to the weight of the displaced liquid in the volume of the body. By placing weights on the balance pan R until the weights bring the balance beam back into equilibrium, the weight of the displaced liquid m is determined. The specific gravity d is equal to the ratio of the masses of the body and water in the volume of the body or the weight of the body and the weight of water in the volume of the body: d = P/P₁. Regarding other methods of measuring specific gravity, see Areometer, Pycnometer.