Density ?

a physical quantity defined, for a homogeneous substance, as the substance’s mass per unit volume. The density

of an inhomogeneous substance is the limit of the ratio of mass to volume when the volume shrinks to the point at which the density is determined.

The ratio of the density of one substance to the density of a reference substance under specified standard physical conditions is called the relative density. For liquids and solids it is usually determined with respect to the density of distilled water at 4°C, and for gases, with respect to the density of dry air or hydrogen under normal conditions. The mean density of a body is defined as the ratio of the body’s mass m to its volume V; that is, ρ = m/V. The unit of density in the International System of Units is kg/m³; in the centimeter-gram-second system it is g/cm³. Other units of density, such as the g/liter and ton/m³, are also widely used.

Densimeters, pycnometers, hydrometers, and hydrostatic weighing are used to measure the density of substances. Other methods of determining density are based on the relation between density and the parameters of state of a substance or on the dependence of the processes occurring in a substance on the substance’s density. Thus, the density of an ideal gas can be calculated from the equation of state ρ = pμ/RT, where ρ is the gas pressure, μ is the molecular weight of the gas, R is the gas constant, and T is the absolute temperature. The density of an ideal gas can also be determined, for example, from the rate of propagation of ultrasound , where β is the adiabatic compressibility of the gas.

The range of values of the density of natural bodies and media is exceptionally broad. For example, the density of the interstellar medium does not exceed 10^-21kg/m³. The mean density of the sun is 1,410 kg/m³, and that of the earth 5,520 kg/m³. The maximum density of metals is 22,500 kg/m³ (osmium). The density of atomic nuclei is 10¹⁷ kg/m³. Finally, the density of neutron stars can apparently reach 10²⁰ kg/m³.

The values of the densities of some widely used substances and materials are presented in Table 1.

For porous and bulk materials a distinction is made between the real density, wherein the voids in the body are not considered, and the apparent density, which is the ratio of the mass of the body to the entire volume occupied by it. Density generally decreases with increasing temperature—as a result of thermal expansion—and increases with rising pressure. Such substances as water, cast iron, and amorphous quartz behave anomalously. For example, the density of water has a maximum value at 4°C and decreases with both increasing and decreasing temperature. During changes in aggregation state of a substance, the density changes discontinuously. Upon a transition from a liquid to a solid state, the density usually increases. Water, however, is an example of a substance that behaves in a different way—its density decreases when it freezes.