Density is the ratio of the mass of a substance to its volume. If two objects of different weights are the same size, the heavier one is more dense. Density is usually measured in grams per cubic centimeter gm/cc.) or pounds per cubic foot (lb/cu. ft.) for solids and liquids and in grams per liter (gm/liter) for gases.
The density of a substance is determined by dividing its mass by the volume that it occupies at a specified temperature and pressure. On a molecular level, density depends both on the mass of the individual molecules composing a substance and on their separation. The heavier or closer together its molecules, the denser the substance.
Density measurements are useful in comparing the relative weights of substances. Mercury, for example, has a density of 13.5 gm/cc. at 25° C. Since the density of water is 1.0 gm/cc. at 25° C., mercury is 13.5 times heavier than water. Furthermore, since each substance has a characteristic density, a substance may in a number of cases be identified by determining its characteristic density.
The ratio of the density of a solid or liquid to the density of pure water at 4° C. is called the specific gravity of that substance. The specific gravity of a gas is obtained by comparing its density with that of air at 0° C. and standard atmospheric pressure.