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/m^{3})
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·L1) 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·L1 (now 0.999 972 kg·L1). 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·m3
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·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.
