WATER PLANT OPERATOR TRAINING PROGRAM
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Although there may be many numbers that express the density of the same substance (depending on the unit used) there is only one relative density associated with each substance (for one particular temperature and pressure). The relative density of a substance is compared against a “standard” density. Specific Gravity Of Solids & Liquids The standard density used for solids and liquids is that of water, which is one g/cm3 at 4 degrees C and a pressure of 101.3 kN/m2 or kilopascals (kpa), the pressure of the atmosphere at sea level. Therefore, the specific gravity of a solid or liquid is the density of that solid or liquid COMPARED TO THE DENSITY OF WATER. It is the ratio of the density of that substance to the density of water. The density of SAE 30 motor oil is: Specific gravity = Oil/water = 0.9 g/cm3/1.0 g/cm3 = 0.90 In other words, specific gravity in this example tells you that oil is only 9/10 as dense as water. Because a cm3 of oil has a mass less than a cm3 of water, oil floats on the surface of water. Specific Gravity of Gases The specific gravity of a gas is usually determined by comparing the density of the gas with the density of air, which is 1.2g/L at a temperature of 20 degrees C and a pressure of 101.3 (kN/m) or (kPa) the pressure of the atmoshere at sea level. For example, the density of chlorine gas is 2.99 g/L. Its specific gravity would be calculated as follows: Specific Gravity = Cl2/air = 2.99 g/L/1.2g/L = 2.49 This tells you that chlorine gas is approximately 2.5 times as dense as air. Therefore, when chlorine gas is introduced into a room it will concentrate at the bottom of the room. This is important to know since chlorine is a toxic gas. |