**Density**,
or **volumic mass**, is a measure of
mass
per unit
volume.
The average density of an object equals its total mass divided by its
total volume. An object made from a comparatively dense material (such
as iron) will have more mass than an equal-sized object made from some
less dense substance (such as aluminium).

A tables of densities of
some materials:

**
Notation,
units, and properties**

The standard symbol used for density is
**ρ**, the *Greek letter* **rho**. So we have

where

**m** is the object's
total mass ([M]; kg)
**V** is the object's
total volume ([L³]; m³)

The **SI** unit of density is the **kilogram
per cubic metre** (kg/m³), but **grams per cubic centimeter**
(g/cm³) and **kilograms per litre** (kg/L) are also commonly used.
Liquid **water** has a density of about 1000 kg/m³ (or 1 kg/L
or 1 g/cm³), solid **iron** has a density of about 8000 kg/m³,
and **air** at room temperature and atmospheric pressure has a
density of about 1.2 kg/m³.

The density of an object or substance
depends on its temperature, with higher temperature usually (but not
always) resulting in lower density. This temperature-density
relationship is captured by the substance's volumetric thermal expansion
coefficient **β**. The density of a gas (and to a much lesser
extent also that of a solid or liquid) further depends on the pressure,
with higher pressure resulting in higher density. The density of a gas
is very dependent on the gas laws.

The density of an object does not have
to be uniform, for instance if the object is composed of different
materials, or if its pressure or temperature is not the same everywhere.
The density of a moving gas may vary if its velocities yield a Mach
number higher than about 0.3. In this case, one defines the density at a
specific point by taking a tiny sphere around that point and dividing
the mass contained within that sphere by the sphere's volume.

##
**Other
units**

In Imperial
units or U.S.
customary units,
the units of
density include pounds per cubic foot (lb/ft³), pounds per cubic yard
(lb/yd³), pounds per cubic inch (lb/in³), ounces per cubic inch (oz/in³),
pounds per gallon (for U.S. or imperial gallons) (lb/gal), pounds per
U.S.
bushel
(lb/bu), in some engineering calculations slugs per cubic foot, and
other less common units.

The maximum density of pure water at a
pressure of one standard atmosphere is 999.861 kg/m³; this occurs
at a temperature of about 3.98°C (277.13 K).

From 1901 to 1964, a litre was defined
as exactly the volume of 1 kg of water at maximum density, and the
maximum density of pure water was 1.000 000 kg/L (now 0.999 972 kg/L).
However, while that definition of the litre was in effect, just as it is
now, the maximum density of pure water was 0.999 972 kg/dm³.
During that period students had to learn the esoteric fact that a cubic
centimeter and a milliliter were slightly different volumes, with 1 mL =
1.000 028 cm³. (often stated as 1.000 027 cm³ in
earlier literature).

**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.
*
***
More Information ...**