Density is the amount of mass an object has per unit volume that the object occupies. The most common units of density include kg/L or kg/m3 in metric units, and lb/gal or lb/ft3 in imperial units.
Another way of expressing density is specific gravity, defined as the ratio of the density of the object to that of water at a given temperature. Specific gravity is thus dimensionless. Substances are denser than water if they have a specific gravity greater than 1, and less dense if it is less than 1. However, it does not follow that water has a specific gravity of 1, since the density of water varies according to the temperature. The use of specific gravity is generally discouraged in present-day scientific and technical writing, with absolute values of density preferred.
- Wile, Dr. Jay L. Exploring Creation With Chemistry. Apologia Educational Ministries, Inc. 1998