Specific gravity and density are related in that specific gravity is a ratio based on a standard density used in science, with water as the base density for liquids and solids, and air or hydrogen gas as the base density for gasses. Density is the average compactness of a substance, measured usually as pounds per cubic foot (lbs/ft^{3}) or kilograms per cubic meter (kg/m^{3}). Dividing the density of a known substance by the reference density of water determines its specific gravity.
An example of the relation between specific gravity and density with solids can be demonstrated by looking at the quality of an element such as gold. Pure gold has a specific gravity of 19.33 and a solid density at a room temperature of 68° Fahrenheit (20° Celsius) of 19.30 grams per cubic centimeter (g/cm^{3}). The reason these numbers appear so similar is due to the reference point of water in calculating specific gravity, with water's density at room temperature being very close to 1 g/cm^{3}. Actually, the density of water is 0.99823 g/cm^{3}, which results in slightly different numbers in specific gravity and density for solids. Expressing gold's specific gravity in more accurate terms as the ratio of gold's density to that of water's density would be 19.30 g/cm^{3} divided by 0.99823 g/cm^{3} to equal a ratio of 19.33/1.
Expressions of specific gravity and density with liquids incorporate Pascal's law, which states that the pressure in a confined, incompressible volume of fluid is the same everywhere. Pressure considerations are also taken into account for the density of gasses using ideal gas law, to produce approximations as to the state of a gas based on pressure, volume, and temperature. With all specific gravity calculations, the reference density of air should be one atmosphere of 14.7 lb/in^{2} (101.3 KPa).
One of the most obvious values of specific gravity and density calculations is that the ratio immediately tells if a material will sink or float in water. Any specific gravity above one is more dense than water, but aluminum has a specific gravity of only 2.7 at room temperature versus heavier metals such as gold. Potentially dangerous gasses used in industry such as silane, which will spontaneously combust in air, has a specific gravity of 1.11. Knowing that it is only slightly heavier than air helps in designing fire suppression systems.
Commercially, specific gravity and density tables for various chemicals have a wide array of uses. In gemology, the specific gravity of a sample gem, also known as relative density, is compared against a high-quality reference to determine its value. Such values are also used to determine eggshell thickness to see if eggs are strong enough to process through the food marketing system, to determine the compression strength of soil in the construction industry, and in the oil industry to determine the energy content of petroleum.
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