Density is mass per volume, weight per volume, or specific gravity, which is the density of a material per the density of water. Metric system densities are usually in the units of mass per volume, such as kg/L (kilogram per liter) or g/cm3 (gram per cubic centimeter). English densities are usually in weight per volume, such as #/gal. (pounds per gallon) or #/ft3 (pound per cubic foot). Specific gravity has no units (!) because it is a comparative measurement. Specific gravity is the density of a material compared to the density of water. Expressing density as specific gravity shows neither system.

We can have fun in a density demonstration by passing a large-grapefruit-sized ball of lead around the class. That size of lead ball weighs about 35 pounds. People do not expect something that compact to weigh so much. One way to think of density is, "How much mass is packed into a volume."

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