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Density and specific gravity have very similar, but not quite identical, definitions.
The specific gravity of a liquid can be determined with a hydrometer, a hollow, sealed, calibrated glass tube. The depth to which the hydrometer sinks is inversely proportional to the specific gravity of the liquid.
In the closeup below, we see that the specific gravity of the blue liquid is 1.016.
Specific gravities can be determined in many other ways including pycnometry (weighing a known volume), and x-ray crystallography
At 4 oC, the density of water is 1.0 g/cm3. Therefore, density and specific gravity have the same numeric value at this temperature.
As we change temperature the volumes of different materials change in different ways. Most, but not all, materials expand (occupying a greater volume) as temperature is raised, meaning that their densities decrease. As the change in volume of water and other materials are not generally equal, the density and specific gravity may no longer be identical as temperature is changed.
For most materials, the volume change going from 4 oC to room temperature (20 oC, typically) is not very large. Therefore, we often use the terms density and specific gravity interchangeably as these values will not differ by more than one or two percent in most cases.
The density or specific gravity can be handy if we know the mass of material that was shipped to us but we need to know the volume...or vice-versa. To interconvert mass, volume and density, we can use any of the following equations which are simply rearranged forms of each other:
A common error in such conversions is to divide instead of multiplying or vice-versa. This doesn't require memorizing the three different versions of the equation. All we have to do is write out the units and make sure that they cancel properly.
For example, if we have 25 g of a material with a density of 0.798 g/cm3, what size container will we require?
Notice that the first two methods give units that make no physical sense and are certainly not volume units. If we always write our units, we will not make a mistake in density/mass/volume calculations.
Densities of some common materials are balsa wood (0.16 g/cm3), pine wood (0.5 g/cm3), gasoline (0.75 g/cm3), soybean oil (0.92 g/cm3), aluminum (2.7 g/cm3)), granite (2.75 g/cm3), lead (11.3 g/cm3), mercury (13.5 g/cm3) and gold (19.3 g/cm3). The densest material on Earth (not counting subatomic particles) is iridium metal (22.65 g/cm3).
As described above, density can be used to help determine volume or mass. Another use for specific gravity is to tell us if the material will sink or float in water or other liquid (assuming that it does not dissolve, of course). For example, a rock with a density of 4.3 g/cm3 will sink in water (density = 1.0 g/cm3), but a piece of plastic with a density of 0.8 g/cm3 will float in water.
Remember the following:
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See also: Baumé scale, concentration, mass units, mole, volume units.
Additional definitions from Google and OneLook.
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