Density

For other meanings of density, see density (disambiguation)

Density (symbol: ρ - Greek: rho) is a measure of mass per unit of volume. The higher an object's density, the higher its mass per volume. The average density of an object equals its total mass divided by its total volume. A denser object (such as iron) will have less volume than an equal mass of some less dense substance (such as water). The SI unit of density is the kilogram per cubic metre (kg/m³)

<math>\rho = \frac{m}{V}<math>

where

ρ is the object's density (measured in kilograms per cubic metre)

m is the object's total mass (measured in kilograms)

V is the object's total volume (measured in cubic metres)

Other units

Many people still use g/cm³ (gram per cubic centimetre) or kg/L (kilograms per litre) to measure density.

1 kg/dm³ = 1000 g/1000cm³ = 1 kg/L.

In Imperial units or U.S. customary unit, the unit of density is the pound/cubic foot.

Formerly mass and volume were linked by defining the gram to be the mass of one cubic centimeter of water at 4°C which meant that water had density 1 kg/litre. However, using one cubic centimetre of water as a standard for one gram is problematic due to the possibility of mass loss from evaporation as well as changes in density with temperature. For this reason alternative definitions of the meter and kilogram have been developed, which can be reproduced more reliably in a laboratory. Because of slight changes in the metre and kilogram due to these new definitions, the density of water at 4°C is not quite exactly 1, but 0.99995 kg/litre. A cubic metre of water thus weighs approximately one metric tonne.

Density of substances

Perhaps the highest density known is reached in neutron star matter. The singularity at the centre of a black hole, according to general relativity, does not have any volume, so its density is undefined.

The densest naturally occurring substance on Earth is Iridium, at about 22.65 kg/litre.

A table of densities of various substances:

Substance	Density (103 kg/m3)
Osmium	22.61
Platinum	21.45
Gold	19.3
Uranium	19.05
Mercury	13.58
Palladium	12.023
Lead	11.34
Silver	10.49
Copper	8.92
Iron	7.87
Tin	7.31
Diamond	3.5
Aluminium	2.70
Magnesium	1.74
Seawater	1.025
Water	1.00
Ethyl alcohol	0.79
Gasoline	0.73
Aerogel	0.003
Air	0.0012

Note the low density of aluminium compared to most other metals. For this reason, aircraft were made of aluminium in the past. Also note that air has a nonzero, albeit small, density. Aerogel is the world's lightest solid.

Table - density of air ρ, Speed of sound in air c,
acoustic impedance Z vs. temperature °C

Impact of temperature
°C	c in m/s	ρ in kg/m³	Z in N·s/m³
- 10	325.4	1.341	436.5
- 5	328.5	1.316	432.4
0	331.5	1.293	428.3
+ 5	334.5	1.269	424.5
+ 10	337.5	1.247	420.7
+ 15	340.5	1.225	417.0
+ 20	343.4	1.204	413.5
+ 25	346.3	1.184	410.0
+ 30	349.2	1.164	406.6

Relative density

Relative density, formerly called specific gravity, is a dimensionless quantity defined as the density of a substance divided by the density of water at standard temperature and pressure. By definition, then, the relative density (or RD) of water is 1, and the RD of osmium is about 22.

See also

• ISO 31: volumic mass
• Dord
• Standard temperature and pressure
• Relative density