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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)

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.

Other units

Density in terms of the SI base units is expressed in kilograms per cubic meter (kg·m-3). Other units fully within the SI include grams per cubic centimetre (g·cm-3) and megagrams per cubic metre (Mg·m-3). 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-1) 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·m-3; this occurs at a temperature of about 3.98 °C (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-1 (now 0.999 972 kg·L-1). 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·dm-3. During that period students had to learn the esoteric fact that a cubic centimetre and a millilitre were slightly different volumes, with 1 mL = 1.000 028 cm³. (often stated as 1.000 027 cm³ in earlier literature).

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 (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 most dense naturally occurring substance on Earth is iridium, at about 22650 kg·m-3.
A table of densities of various substances:

Substance    Density in kg·m-3
Iridium      22650
Osmium     22610
Platinum    21450
Gold         19300
Tungsten   19250
Uranium    19050
Mercury     13580
Palladium  12023
Lead         11340
Silver        10490
Copper      8960
Iron          7870
Steel         7850
Tin           7310
Titanium   4507
Diamond   3500
Basalt       3000
Granite     2700
Aluminium 2700
Graphite   2200
Magnesium 1740
PVC         1300
Seawater  1025
Water      1000
Ice          917
Polyethylene      910
Ethyl alcohol      790
Gasoline            730
Liquid Hydrogen  68
Aerogel     3
any gas    0.0446 times the average molecular mass, hence between 0.09 and ca. 13.1 (at room temperature and pressure)
For example air    1.2

Density of air ρ vs. temperature °C

T in °C    ρ in kg·m-3
- 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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