# 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.

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## density

density, ratio of the mass of a substance to its volume, expressed, for example, in units of grams per cubic centimeter or pounds per cubic foot. The density of a pure substance varies little from sample to sample and is often considered a characteristic property of the substance. Most substances undergo expansion when heated and therefore have lower densities at higher temperatures. Many substances, especially gases, can be compressed into a smaller volume by increasing the pressure acting on them. For these reasons, the temperature and pressure at which the density of a substance is measured are usually specified. The density of a gas is often converted mathematically to what it would be at a standard temperature and pressure (see STP). Water is unusual in that it expands, and thus decreases in density, as it is cooled below 3.98°C (its temperature of maximum density). Density often is taken as an indication of how "heavy" a substance is. Iron is denser than cork, since a given volume of iron is more massive (and weighs more) than the same volume of cork. It is often said that iron is "heavier" than cork, although a large volume of cork obviously can be more massive and thus be heavier (i.e., weigh more) than a small volume of iron. See specific gravity.

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## density

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.

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## density

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.

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## density

density
1. A measure of the amount of information in a given dimension of a storage medium. The density of information on a disk is almost always a fixed number of bits per sector, sectors per track, and tracks per disk. For magnetic tape it is the amount of information recorded per unit length of tape, usually in bits per inch or bits per millimeter. In general the number of flux reversals per inch (or per mm) is different because of redundancy in the coding. The density is stated for a single track. A tape transport can often read tapes with different densities under program control.

2. See packing density.

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## density

density Ratio of mass to volume for a given substance usually expressed in SI units as (kg/m3). It is an indication of the concentration of particles within a material. The density of a solid or liquid changes little over a wide range of temperatures and pressures. Relative density (r.d.) is the ratio of the density of one substance to that of a reference substance (usually water) at the same temperature and pressure. The density of a gas depends on both pressure and temperature.

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## density

density The degree of aggregation, concentration, or crowding within a defined geographical or social space, measured in different ways. The population density of an area is measured by the number of persons usually resident in the area or, sometimes, by the number of persons working in an area (the day-time population density). Trade-union membership density is the percentage of all workers at a particular workplace, or in a company, who are union members.

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## density

density Mass per unit volume, expressed in SI units as kilograms per cubic metre (kg/m3).

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## density

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