Density

Density is defined as the mass of a unit volume of some material. The term unit volume means the amount contained in one volumetric unit of measurement: one cubic foot, one liter, or one milliliter, for example. The density of pure iron, for instance, is 7.87 grams per cubic centimeter (7.87 g/cm³). That statement means that one cubic centimeter (1 cm³) of iron has a mass of 7.87 grams.

Density is an important measurement because it allows for the comparison of the heaviness of two materials. Rather than asking, "Which is heavier: iron or Styrofoam™?," the proper question to ask is: "Which is more dense: iron or Styrofoam™?" This question is more appropriate because it asks about a comparison between equal amounts of two different materials.

Specific gravity

Another form of measurement closely associated with density is specific gravity. The specific gravity of a material is the density of that material compared to the density of some standard. For solids and liquids, the most common standard is water, whose density is 1.00 gram per cubic centimeter. The specific gravity of iron, then, is its density (7.87 grams per cubic centimeter) divided by the density of water (1.00 gram per cubic centimeter). You can see that the numerical value for the specific gravity of a solid or liquid is always the same as that of its density.

The reason is that the divisor in every case is 1 gram per cubic centimeter, the density of water. For iron, the specific gravity is 7.87. The only difference between density and specific gravity for solids and liquids is that specific gravity has no label. In dividing 7.87 grams per cubic centimeter by 1.00 gram per cubic centimeter, the labels divide out (cancel), leaving only the number.

The specific gravity of gases is somewhat more difficult since the most common standards are air (density = 1.293 grams per cubic centimeter) or hydrogen (density = 0.0899 gram per cubic centimeter). The specific gravity of oxygen using air as a standard, then, is its density (1.429 grams per cubic centimeter) divided by the density of air (1.293 grams per cubic centimeter), or 1.105. Using hydrogen as a standard, the specific gravity of oxygen is 1.429 grams per cubic centimeter ÷ 0.0899 gram per cubic centimeter, or 15.9.

Density

The density of an object is the mass of the object divided by its volume. For example, imagine you have two boxes, not necessarily of the same size. You are told that one is filled with feathers and the other is filled with cement. You can tell when you pick up the boxes, without looking inside, which is the box filled with cement and which is the box filled with feathers. The box filled with cement will be heavier for its size. It would take a large box of feathers to equal the weight of a small box of cement because the box of cement will always have a higher density.

Density does not depend on how much of the material there is. One pound of cement has the same density as one ton of cement. Both the mass and the volume are properties that depend on how much of the material an object has. Dividing the mass by the volume has the effect of canceling the amount of material. If you are buying a piece of gold jewelry, you can (in theory) tell if the piece is solid gold or gold plated steel by measuring the mass and volume of the piece and computing its density. Does it have the density of gold? The mass is usually measured in kilograms or grams and the volume is usually measured in cubic meters or cubic centimeters, so the density is measured in either kilograms per cubic meter or in grams per cubic centimeter.

The density of a material is also often compared to the density of water to give the material’s specific gravity. Typical rocks near the surface of the Earth will have specific gravities of 2 to 3, meaning they have densities of two to three times the density of water. The entire Earth has a density of about five times the density of water. Therefore the center of the Earth must be a high density material such as nickel or iron. The density provides an important clue to the interior composition of objects, such as the Earth and planets, that we cannot take apart or look inside. In cosmology, the density of the universe is an important parameter in determining the long-term fate of the universe. Since the universe is finite in size and contains a finite amount of mass and energy, it has a definite overall density, just like any smaller object.

density Mass per unit volume, expressed in SI units as kilograms per cubic metre (kg/m³). It is determined directly, or indirectly using gravity or seismic velocity measurements. Typical densities in unconsolidated, wet sediments range between 1200 and 2600 kg/m³, and 1000 to 2000 kg/m³ if dry. Consolidated sediments range between 1600 and 3200 kg/m³. (See also NAFE—DRAKE RELATIONSHIPS for seismic determinations of the density of sediments.) Basic igneous rocks range between 2300 and 3170 kg/m³, and metamorphic rocks are mostly in the range 2400–3100 kg/m³, with eclogite being one of the denser of the commoner metamorphic rocks, between 3200 and 3540 kg/m³. The density of the upper mantle increases with depth from about 3330 to 4000 kg/m³, below which the lower mantle systematically increases to about 5400 kg/m³ above the core. The outer core density is about 10 100 to 12 100 kg/m³; the inner core is more uniform, with a density of about 13 000 kg/m³.

Density

The density of an object is defined simply as the mass of the object divided by the volume of the object. For a concrete example, imagine you have two identical boxes. You are told that one is filled with feathers and the other is filled with cement. You can tell when you pick up the boxes, without looking inside, which is the box filled with cement and which is the box filled with feathers. The box filled with cement will be heavier. It would take a very large box of feathers to equal the weight of a small box of cement because the box of cement will always have a higher density.

Density is a property of the material that does not depend on how much of the material there is. One pound of cement has the same density as one ton of cement. Both the mass and the volume are properties that depend on how much of the material an object has. Dividing the mass by the volume has the effect of canceling the amount of material. If you are buying a piece of gold jewelry, you can tell if the piece is solid gold or gold plated steel by measuring the mass and volume of the piece and computing its density. Does it have the density of gold? The mass is usually measured in kilograms or grams and the volume is usually measured in cubic meters or cubic centimeters, so the density is measured in either kilograms per cubic meter or in grams per cubic centimeter.

The density of a material is also often compared to the density of water to give the material's specific gravity. Typical rocks near the surface of Earth will have specific gravities of 2 to 3, meaning they have densities of two to three times the density of water. The entire Earth has a density of about five times the density of water. Therefore the center of Earth must be a high density material such as nickel or iron . The density provides an important clue to the interior composition of objects, such as Earth and planets, that we can't take apart or look inside.

den·si·ty / ˈdensitē/ • n. (pl. -ties) the degree of compactness of a substance: a reduction in bone density. ∎ Comput. a measure of the amount of information on a storage medium (tape or disk). For magnetic tape it is the amount of information recorded per unit length of tape (bits per inch or millimeter); for a disk, a fixed number of bits per sector, sectors per track, and tracks per disk: chip density doubles every eighteen months | [in comb.] a low-density 5.25-inch floppy disk a drive capable of handling high-density 1.44 megabyte disks. ∎  Physics degree of consistency measured by the quantity of mass per unit volume. ∎  the opacity of a photographic image. ∎  the quantity of people or things in a given area or space: areas of low population density | a density of 10,000 per square mile.