The National Science Resources Center says that density is one of the characteristic properties of an object, which, when compared to the densities of other known objects, allows an investigator to identify the materials from which it is made. While several materials may have similar densities, this property allows an individual to rule out unlikely candidates and use further tests to confirm identification.
Reference.com defines density as the ratio of an object's volume to its mass. Several factors affect density and it sometimes varies slightly from one sample to the next. Density varies with temperature as many materials expand upon heating. In addition, the density of a gas increases as pressure increases. When speaking of the density of a gas, scientists often specify the standard temperature and pressure to eliminate these variables. Managing these conditions is essential if one is using density to identify a gas.
Specific gravity relates to density. It is the ratio of the material as compared to water and carries no units. About.com says objects with specific gravities greater than 1 sink in water, while those with a specific gravity of less than 1 float. This quick test of nonsoluble items helps to eliminate possible choices when identifying a solid.