Density

Density of a substance - the mass of a given substance in a unit of its volume. Density is determined by the mass of its molecules and the density of their arrangement (their number in a unit volume). In solids and liquids, this density varies, and there is no definite dependence between molecular weight and density. In the state of an ideal gas, the number of molecules of all substances in a unit volume is the same under equal conditions; in this state, density is proportional to molecular weight. The density of solids and liquids is expressed in g/cm³, since the unit of volume is considered to be 1 cm³. The density of gases and vapors, due to its small magnitude, is expressed as ratios: g/L or g/m³. The density of water, equal in the absolute system of physical units (CGS) to the mass of 1 cm³ of water at the temperature of its greatest density (at 3.99°C), is taken as the unit. The ratio of the density of any body to the density of water at this temperature is called its specific gravity and is an abstract number; density, however, is a named number, as this concept has dimensionality. The density of solids and liquids is measured with an areometer, a pycnometer (see) or by the method of hydrostatic weighing.