Different materials usually have different densities, so density is an important concept regarding buoyancy, metal purity and packaging.
In some cases density is expressed as the dimensionless quantities specific gravity or relative density, in which case it is expressed in multiples of the density of some other standard material, usually water or air.
Archimedes knew that the irregularly shaped wreath could be crushed into a cube whose volume could be calculated easily and compared with the weight; but the king did not approve of this.
Baffled, Archimedes took a bath and observed from the rise of the water upon entering that he could calculate the volume of the crown through the displacement of the water. Allegedly, upon this discovery, he went running naked though the streets shouting, "Eureka! Eureka!" (Greek "I found it"). As a result, the term "eureka" entered common parlance and is used today to indicate a moment of enlightenment.
This story first appeared in written form in Vitruvius' books of architecture, two centuries after it supposedly took place. Some scholars have doubted the accuracy of this tale, saying among other things that the method would have required precise measurements that would have been difficult to make at the time.
If the body is inhomogeneous, the density is a function of the coordinates , where is elementary volume with coordinates . The mass of the body then can be expressed as
where the integration is over the volume of the body V.
A very common instrument for the direct measurement of the density of a liquid is the hydrometer, which measures the volume displaced by an object of known mass. A common laboratory device for measuring fluid density is a pycnometer; a related device for measuring the absolute density of a solid is a gas pycnometer. Another instrument used to determine the density of a liquid or a gas is the digital density meter - based on the oscillating U-tube principle.
The density of a solid material can be ambiguous, depending on exactly how its volume is defined, and this may cause confusion in measurement. A common example is sand: if gently filled into a container, the density will be low; when the same sand is compacted into the same container, it will occupy less volume and consequently exhibit a greater density. This is because sand, like all powders and granular solids contains a lot of air space in between individual grains; this overall density is called the bulk density, which differs significantly from the density of an individual grain of sand.
Metric units outside the SI
These are numerically equivalent to kg/L (1 kg/L = 1 kg/dm³ = 1 g/cm³ = 1 g/mL).
The effect of pressure and temperature on the densities of liquids and solids is small so that a typical compressibility for a liquid or solid is 10–6 bar–1 (1 bar=0.1 MPa) and a typical thermal expansivity is 10–5 K–1.
|Temp (°C)||Density (g/cm3)|
| The density of water in grams per cubic centimeter |
at various temperatures in degrees Celsius
The values below 0 °C refer to supercooled water.
Water - Density and Specific Weight
See Water Density
|T in °C||ρ in kg/m3 (at 1 atm)|
|Material||ρ in kg/m3||Notes|
|Interstellar medium||10-25 − 10-15||Assuming 90% H, 10% He; variable T|
|Earth's atmosphere||1.2||At sealevel|
|Aerogel||1 − 2|
|Styrofoam||30 − 120||From|
|Cork||220 − 260||From|
|Plastics||850 − 1400||For polypropylene and PETE/PVC|
|The Earth||5515.3||Mean density|
|Copper||8920 − 8960||Near room temperature|
|Lead||11340||Near room temperature|
|The Inner Core||~13000||As listed in Earth|
|Uranium||19100||Near room temperature|
|Iridium||22500||Near room temperature|
|The core of the Sun||~150000|
|Atomic nuclei||~3 × 1017||As listed in neutron star|
|Neutron star||8.4 × 1016 − 1 × 1018|
|Black hole||2 × 1030||Mean density inside the Schwarzschild radius of an earth-mass black hole (theoretical)|