Specific gravity or relative density of a substance is understood to be the relationship between its density and the density of a reference substance. In general, water at a temperature of 4 degrees Celsius, which is the point at which it has the greatest density, is considered as the reference for the relative density of liquid substances. For gases, air at room temperature, at 20 degrees Celsius, and at a pressure of one atmosphere, is usually considered as a reference. When referring to a relative density value for gases, the temperature and pressure considered must be indicated. Because it is defined as a ratio of the same magnitude, specific gravity or relative density is a parameter that has no units.
By definition, the relative density of water at a temperature of 4 degrees Celsius is 1. The relative density of ethyl alcohol is 0.78; the relative density of urine varies between 1.003 and 1.035 and that of blood is 1.060.
Fountain
Resnick, R.; Halliday, D.; Krane, KS Physics. Vol. 1 . 4th edition in English; in Spanish, 3rd edition. Continental Publishing Company, Mexico, 2001.
