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# Density

Density (symbol: ρ - Greek: rho) (ISO 31: volumic mass) 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).

Again,

where ρ equals density, m equals total mass, and V equals volume.

In the SI system of units, density is measured as kg/m3 (kilogram per cubic metre), but many people use the more convenient g/cm3 (gram per cubic centimetre) or kg/L (kilograms per litre).

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 centimeter 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 meter of water thus weighs approximately one metric tonne.

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 would be seen as either infinite or non-existent.

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)
Osmium22.61
Platinum21.09
Gold19.3
Uranium19.05
Mercury13.58
Silver10.49
Copper8.92
Iron7.87
Tin7.31
Diamond3.5
Aluminium2.70
Magnesium1.74
Seawater1.025
Water1.00
Ethyl alcohol0.79
Gasoline0.68
Aerogel0.003
Air0.0012

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.

Density may denote how much of a certain substance, object or occurrence is present per unit area or volume. Often used is population density, meaning how many people per square kilometre (or square mile) on average live in an area.

The density of discrete entities such as people is difficult to characterise as a continuous quantity.

Geographers and mathematicians have made a number of attempts to formalize the concept of population density.

Charge density is the electric charge per unit area or unit volume.

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