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What are density and specific gravity?
Get a jar. Add a handful of gold nuggets, 1 cup of cooking oil, and fill it up with water. Put a lid on it, shake briskly, and let sit for 37 seconds. Record what happens. Shake it up and try again. Did you get the same response? Why does the gold always settle to the bottom and the oil float to the top? To understand we have to look inside matter and see how it is put together.
Imagine one (1) cubic centimeter of water: if pure and at "standard temperature and pressure" it will have a mass of one (1) gram. The size (1 cm3) is a volume and represents "how big." The mass represents "how much."
Now imagine a cubic centimeter of pure gold, which has a mass of 19.3 grams. The "how big" is the same, but the "how much" has increased by almost 20 times. What about the oil? A cubic centimeter actually has a mass less than 1 gram. For oil, the "how much" is less. Why aren't all 3 substances the same? Clearly there is another factor involved here which relates to "how tightly is it packed at the atomic level?"
We call this new factor "density." Density is calculated by dividing the "how much" by the "how big," and is expressed in grams per cubic centimeter.
How tightly packed = how much / how big
or
Density = Mass / Volume
Specific gravity is the density of a substance divided by the density of water. Since (at standard temperature and pressure) water has a density of 1 gram/cm3, and since all of the units cancel, specific gravity is usually very close to the same value as density (but without any units).
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