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Density is a measure of mass per volume.

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Density is a measure of mass per 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 less volume than an object of equal mass made from some less dense substance (such as water: see image right). The SI unit of density is the kilogram per cubic metre (kg/m3)

The average density of an object equals its total mass divided by its total volume. Here we see water.


? is the object's density (measured in kilograms per cubic meter)
m is the object's total mass (measured in kilograms)
V is the object's total Volume (measured in cubic meters)

Under specified conditions of temperature and pressure, the 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.

Also, in terms of candy making, density is affected by the melting and cooling processes. Loose granular sugar, like sand, contains a lot of air and is not tightly packed, but when it has melted and starts to boil, the sugar loses its granularity and entrained air and becomes a fluid. When you mold it to make a smaller, compacted shape, the syrup tightens up and loses more air. As it cools, it contracts and gains moisture, making the already heavy candy even more dense.

A more theoretical definition is also available. Density can be calculated based on crystallographic information and molar mass:


M is molar mass
N is the number of atoms in a unit cell
L is Loschmidt or Avogadro's number
a, b, c are the lattice parameters

The density with respect to temperature, T, has the following relation:


C is the coefficient of cubic expansion.

Experimentally density can be found by measuring the dry weight (Wd ), the wet weight (Ww) and submersed weight (Ws), usually in water.

Other units

Density in terms of the SI base units is expressed in kilograms per cubic meter (kg/m3). Other units fully within the SI include grams per cubic centimeter (g/cm3) and megagrams per cubic metre (Mg/m3). Since both the litre and the tonne or metric ton are also acceptable for use with the SI, a wide variety of units such as kilograms per litre (kg/L) are also used. 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 999.861kg/m3; this occurs at a temperature of about 3.98 ºC (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).

Density will determine the 'order' in which each substance will appear in a bottle. For example, if substance A has a density of .64g/cm3, and Substance B has a density of .84g/cm3, Substance A will be above Substance B in a container due to the simple fact that its density is lower. One example of this is oil and water, where the oil will remain above.

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.

For a rectagular solid, the formula Mass / (Length x Width x Height) can be used. For an irregularly shaped solid, Displacement (fluid) can be used in place of L x W x H.

Density of substances.

Perhaps the highest density known is reached in neutron star matter (see neutronium). 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 appears to be iridium, at about 22650 kg/m3. However, because this calculation requires a strong theoretical basis, and the difference between iridium and osmium is so small, definitively stating one or the other is more dense is not possible at this time.

A table of masses of various substances:

Substance Density in kg/m3 Particles per cubic metre
iridium 22650 1.06 ×1029
osmium 22610 7.16 ×1028
platinum 21450 6.62 ×1028
gold (0ºC) 19300 5.90 ×1028
Tungsten 19250 6.31 ×1028
uranium 19050 4.82 ×1028
Mercury 13580 4.08 ×1028
Palladium 12023 6.8 ×1028
Lead 11340 3.3 ×1028
Silver 10490 5.86 ×1028
Copper 8960 8.49 ×1028
iron 7870 8.49 ×1028
Steel 7850
Tin 7310 3.71 ×1028
titanium 4507 5.67 ×1028
diamond 3500 1.75 ×1029
basalt 3000
Granite 2700
aluminium 2700 6.03 ×1028
Graphite 2200 1.10 ×1029
magnesium 1740 4.31 ×1028
PVC 1300
Seawater (15ºC) 1025
water (25 ºC) 998 3.34 ×1028
ice (0ºC) 917 3.07 ×1028
Polyethylene 910
Ethyl alcohol 790 1.03 ×1028
Gasoline 730
Liquid Hydrogen 68 4.06 ×1028
Aerogel 3
any gas 0.0446 times the average molecular mass (in g/mol), hence between 0.09 and ca. 13.1 (at 0ºC and 1 atm)
For example air (0º), (25º) 1.29, 1.17
Density of air ? vs. temperature ºC
T in ºC ? in kg/m3
- 10 1.341
- 5 1.316
0 1.293
+ 5 1.269
+ 10 1.247
+ 15 1.225
+ 20 1.204
+ 25 1.184
+ 30 1.164

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.

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