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)

Various types of density

Under specified conditions of temperature and pressure, density of a fluid is defined as described above. However, the density of a solid material can be different, depending on exactly how it is defined. Take sand for example. If you gently fill a container with sand, and divide the mass of sand by the container volume you get a value termed loose bulk density. If you took this same container and tapped on it repeatedly, allowing the sand to settle and pack together, and then calculate the results, you get a value termed tapped or packed bulk density. Tapped bulk density is always greater than or equal to loose bulk density. In both types of bulk density, some of the volume is taken up by the spaces between the grains of sand. If you are interested in the density of the grain of sand itself you need to measure either envelope density or absolute density.

Other units

Some people still use the old g/cm³ (gram per cubic centimetre) or kg/L (kilograms per litre) to measure density,
instead of the correct kg/m³.

1000 kg/m³ = 1 kg/dm³ = kg/L or 1 g/cm³ = 1 g/mL.

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 centimetre of water at 3.98 °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 very slight changes in the metre and kilogram due to these new definitions, the density of clean water at 3.98 °C is very close to exactly 1 kg/litre. A cubic metre of water thus weighs one metric tonne.

Measurement of density

A common device for measuring fluid density is a pycnometer. A device for measuring absolute density of a solid is a gas pycnometer.

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 is 22,650 kg/m³ (or as not SI unit: 22.65 kg/dm³ or 22.65 g/cm³).

A table of densities of various substances:

See also density of gas

Note the low density of aluminium compared to most other metals. For this reason, aircraft are made of aluminium. 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

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

External link

• Conversion: density - Imperial and US customary units to metric units (http://www.sengpielaudio.com/ConvDens.htm)

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