Specific gravity is a ratio based on a standard density used in science, with water as the basic density for liquids and solids and air or hydrogen for gases. Density is the average firmness of a substance. The ratio immediately indicates whether a material will sink or float in water.
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 basic density for liquids and solids and air or hydrogen as the basic density for gases. Density is the average firmness of a substance, usually measured as pounds per cubic foot (lbs/ft3) or kilograms per cubic meter (kg/m3). Dividing the density of a known substance by the reference density of water determines its specific gravity.
An example of the relationship 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/cm3). The reason these numbers appear so similar is due to the reference point of water in calculating specific gravity, with the density of water at room temperature very close to 1 g/cm3. In reality, the density of water is 0.99823 g/cm3, which results in slightly different numbers in specific gravity and density for solids. Expressing the specific gravity of gold in more accurate terms such as the ratio of the density of gold to that of water would be 19.30 g/cm3 divided by 0.99823 g/cm3 to give 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 gas density using the ideal gas law to produce approximations about the state of a gas based on pressure, volume and temperature. With all specific gravity calculations, the reference density of air should be an atmosphere of 14.7 lb/in2 (101.3 KPa).
One of the most obvious values of specific gravity and density calculations is that the ratio immediately indicates whether a material will sink or float in water. Any specific gravity above one is denser than water, but aluminum has a specific gravity of only 2.7 at room temperature compared to heavier metals like gold. Potentially dangerous gases used in industry such as silane, which burns spontaneously in air, have a specific gravity of 1.11. Knowing that it is only slightly heavier than air helps in the design of fire systems.
Commercially, density and specific gravity tables for various chemicals have a wide variety of uses. In gemology, a specimen gem’s specific gravity, also known as its relative density, is compared to a high-quality reference to determine its value. These values are also used to determine eggshell thickness to see if the eggs are strong enough to be processed through the food marketing system, to determine the compressive strength of soil in the construction industry and the oil industry to determine the energy content of oil.
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