Density
The density, or more precisely, the volumetric mass density, of a substance is its mass per unit volume. The symbol most often used for density is ρ (the lower case Greek letter rho), although the Latin letter D can also be used. Mathematically, density is defined as mass divided by volume:^{[1]}
 \rho = \frac{m}{V},
where ρ is the density, m is the mass, and V is the volume. In some cases (for instance, in the United States oil and gas industry), density is loosely defined as its weight per unit volume,^{[2]} although this is scientifically inaccurate – this quantity is more specifically called specific weight.
For a pure substance the density has the same numerical value as its mass concentration. Different materials usually have different densities, and density may be relevant to buoyancy, purity and packaging. Osmium and iridium are the densest known elements at standard conditions for temperature and pressure but certain chemical compounds may be denser.
To simplify comparisons of density across different systems of units, it is sometimes replaced by the dimensionless quantity "relative density" or "specific gravity", i.e. the ratio of the density of the material to that of a standard material, usually water. Thus a relative density less than one means that the substance floats in water.
The density of a material varies with temperature and pressure. This variation is typically small for solids and liquids but much greater for gases. Increasing the pressure on an object decreases the volume of the object and thus increases its density. Increasing the temperature of a substance (with a few exceptions) decreases its density by increasing its volume. In most materials, heating the bottom of a fluid results in convection of the heat from the bottom to the top, due to the decrease in the density of the heated fluid. This causes it to rise relative to more dense unheated material.
The reciprocal of the density of a substance is occasionally called its specific volume, a term sometimes used in thermodynamics. Density is an intensive property in that increasing the amount of a substance does not increase its density; rather it increases its mass.
Contents
 History 1

Measurement of density 2
 Homogeneous materials 2.1
 Heterogeneous materials 2.2
 Noncompact materials 2.3
 Changes of density 3
 Density of solutions 4

Densities 5
 Water 5.1
 Air 5.2
 Various materials 5.3
 Others 5.4
 Common units 6
 See also 7
 References 8
 External links 9
History
In a wellknown but probably apocryphal tale, Archimedes was given the task of determining whether King Hiero's goldsmith was embezzling gold during the manufacture of a golden wreath dedicated to the gods and replacing it with another, cheaper alloy.^{[3]} Archimedes knew that the irregularly shaped wreath could be crushed into a cube whose volume could be calculated easily and compared with the mass; but the king did not approve of this. Baffled, Archimedes is said to have taken an immersion bath and observed from the rise of the water upon entering that he could calculate the volume of the gold wreath through the displacement of the water. Upon this discovery, he leapt from his bath and ran naked through the streets shouting, "Eureka! Eureka!" (Εύρηκα! Greek "I have found it"). As a result, the term "eureka" entered common parlance and is used today to indicate a moment of enlightenment.
The story first appeared in written form in Vitruvius' books of architecture, two centuries after it supposedly took place.^{[4]} Some scholars have doubted the accuracy of this tale, saying among other things that the method would have required precise measurements that would have been difficult to make at the time.^{[5]}^{[6]}
From the equation for density (ρ = m / V), mass density has units of mass divided by volume. As there are many units of mass and volume covering many different magnitudes there are a large number of units for mass density in use. The SI unit of kilogram per cubic metre (kg/m^{3}) and the cgs unit of gram per cubic centimetre (g/cm^{3}) are probably the most commonly used units for density.1,000 kg/m^{3} equals 1 g/cm^{3}. (The cubic centimeter can be alternately called a millilitre or a cc.) In industry, other larger or smaller units of mass and or volume are often more practical and US customary units may be used. See below for a list of some of the most common units of density.
Measurement of density
Homogeneous materials
The density at all points of a homogeneous object equals its total mass divided by its total volume. The mass is normally measured with a scale or balance; the volume may be measured directly (from the geometry of the object) or by the displacement of a fluid. To determine the density of a liquid or a gas, a hydrometer, a dasymeter or a Coriolis flow meter may be used, respectively. Similarly, hydrostatic weighing uses the displacement of water due to a submerged object to determine the density of the object.
Heterogeneous materials
If the body is not homogeneous, then its density varies between different regions of the object. In that case the density around any given location is determined by calculating the density of a small volume around that location. In the limit of an infinitesimal volume the density of an inhomogeneous object at a point becomes: ρ(\vec{r}) = dm/dV, where dV is an elementary volume at position r. The mass of the body then can be expressed as
 m = \int_V \rho(\vec{r})\,dV.
Noncompact materials
In practice, bulk materials such as sugar, sand, or snow contain voids. Many materials exist in nature as flakes, pellets, or granules.
Voids are regions which contain something other than the considered material. Commonly the void is air, but it could also be vacuum, liquid, solid, or a different gas or gaseous mixture.
The bulk volume of a material—inclusive of the void fraction—is often obtained by a simple measurement (e.g. with a calibrated measuring cup) or geometrically from known dimensions.
Mass divided by bulk volume determines bulk density. This is not the same thing as volumetric mass density.
To determine volumetric mass density, one must first discount the volume of the void fraction. Sometimes this can be determined by geometrical reasoning. For the closepacking of equal spheres the nonvoid fraction can be at most about 74%. It can also be determined empirically. Some bulk materials, however, such as sand, have a variable void fraction which depends on how the material is agitated or poured. It might be loose or compact, with more or less air space depending on handling.
In practice, the void fraction is not necessarily air, or even gaseous. In the case of sand, it could be water, which can be advantageous for measurement as the void fraction for sand saturated in water—once any air bubbles are thoroughly driven out—is potentially more consistent than dry sand measured with an air void.
In the case of noncompact materials, one must also take care in determining the mass of the material sample. If the material is under pressure (commonly ambient air pressure at the earth's surface) the determination of mass from a measured sample weight might need to account for buoyancy effects due to the density of the void constituent, depending on how the measurement was conducted. In the case of dry sand, sand is so much denser than air that the buoyancy effect is commonly neglected (less than one part in one thousand).
Mass change upon displacing one void material with another while maintaining constant volume can be used to estimate the void fraction, if the difference in density of the two voids materials is reliably known.
Changes of density
In general, density can be changed by changing either the pressure or the temperature. Increasing the pressure always increases the density of a material. Increasing the temperature generally decreases the density, but there are notable exceptions to this generalization. For example, the density of water increases between its melting point at 0 °C and 4 °C; similar behavior is observed in silicon at low temperatures.
The effect of pressure and temperature on the densities of liquids and solids is small. The compressibility for a typical liquid or solid is 10^{−6} bar^{−1} (1 bar = 0.1 MPa) and a typical thermal expansivity is 10^{−5} K^{−1}. This roughly translates into needing around ten thousand times atmospheric pressure to reduce the volume of a substance by one percent. (Although the pressures needed may be around a thousand times smaller for sandy soil and some clays.) A one percent expansion of volume typically requires a temperature increase on the order of thousands of degrees Celsius.
In contrast, the density of gases is strongly affected by pressure. The density of an ideal gas is
 \rho = \frac {MP}{RT}, \,
where M is the molar mass, P is the pressure, R is the universal gas constant, and T is the absolute temperature. This means that the density of an ideal gas can be doubled by doubling the pressure, or by halving the absolute temperature.
In the case of volumic thermal expansion at constant pressure and small intervals of temperature the temperature dependence of density is :
 \rho = \frac }
where \rho_{T_0} is the density at a reference temperature, \alpha is the thermal expansion coefficient of the material at temperatures close to T_0.
Density of solutions
The density of a solution is the sum of mass (massic) concentrations of the components of that solution.
Mass (massic) concentration of each given component ρ_{i} in a solution sums to density of the solution.
 \rho = \sum_i \varrho_i \,
Expressed as a function of the densities of pure components of the mixture and their volume participation, it allows the determination of excess molar volumes:
 \rho = \sum_i \rho_i \frac{V_i}{V}\,= \sum_i \rho_i \varphi_i =\sum_i \rho_i \frac{V_i}{\sum_i V_i + \sum_i V_i^{E}}
provided that there is no interaction between the components.
Knowing the relation between excess volumes and activity coefficients of the components, one can determine the activity coefficients.
 \bar{V^E}_i= RT \frac{\partial (ln(\gamma_i))}{\partial P}
Densities
Water
Temp (°C)^{[note 1]}  Density (kg/m^{3}) 

−30  983.854 
−20  993.547 
−10  998.117 
0  999.8395 
4  999.9720 
10  999.7026 
15  999.1026 
20  998.2071 
22  997.7735 
25  997.0479 
30  995.6502 
40  992.2 
60  983.2 
80  971.8 
100  958.4 
Notes:

Air
T (°C)  ρ (kg/m^{3}) 

−25  1.423 
−20  1.395 
−15  1.368 
−10  1.342 
−5  1.316 
0  1.293 
5  1.269 
10  1.247 
15  1.225 
20  1.204 
25  1.184 
30  1.164 
35  1.146 
Various materials
Material  ρ (kg/m^{3})^{[note 1]}  Notes 

Helium  0.179  
Aerographite  0.2  ^{[note 2]}^{[7]}^{[8]} 
Metallic microlattice  0.9  ^{[note 2]} 
Aerogel  1.0  ^{[note 2]} 
Air  1.2  At sea level 
Tungsten hexafluoride  12.4  One of the heaviest known gases at standard conditions 
Liquid hydrogen  70  At approx. –255 °C 
Styrofoam  75  Approx.^{[9]} 
Cork  240  Approx.^{[9]} 
Pine  373  ^{[10]} 
Lithium  535  
Wood  700  Seasoned, typical^{[11]}^{[12]} 
Oak  710  ^{[10]} 
Potassium  860  ^{[13]} 
Sodium  970  
Ice  916.7  At temperature < 0 °C 
Water (fresh)  1,000  At 4 °C, the temperature of its maximum density 
Water (salt)  1,030  
Nylon  1,150  
Plastics  1,175  Approx.; for polypropylene and PETE/PVC 
Tetrachloroethene  1,622  
Magnesium  1,740  
Beryllium  1,850  
Glycerol  1,261  ^{[14]} 
Concrete  2,000  ^{[15]} 
Silicon  2,330  
Aluminium  2,700  
Diiodomethane  3,325  liquid at room temperature 
Diamond  3,500  
Titanium  4,540  
Selenium  4,800  
Vanadium  6,100  
Antimony  6,690  
Zinc  7,000  
Chromium  7,200  
Tin  7,310  
Manganese  7,325  Approx. 
Iron  7,870  
Niobium  8,570  
Brass  8,600  ^{[15]} 
Cadmium  8,650  
Cobalt  8,900  
Nickel  8,900  
Copper  8,940  
Bismuth  9,750  
Molybdenum  10,220  
Silver  10,500  
Lead  11,340  
Thorium  11,700  
Rhodium  12,410  
Mercury  13,546  
Tantalum  16,600  
Uranium  18,800  
Tungsten  19,300  
Gold  19,320  
Plutonium  19,840  
Platinum  21,450  
Iridium  22,420  
Osmium  22,570  
Notes:

Others
Entity  ρ (kg/m^{3})  Notes 

Interstellar medium  ×10^{−19} 1  Assuming 90% H, 10% He; variable T 
The Earth  5,515  Mean density.^{[16]} 
The inner core of the Earth  13,000  Approx., as listed in Earth.^{[17]} 
The core of the Sun  33,000–160,000  Approx.^{[18]} 
Supermassive black hole  ×10^{5} 9 
Density of a 4.5millionsolarmass black hole Event horizon radius is 13.5 million km. 
White dwarf star  ×10^{9} 2.1  Approx.^{[19]} 
Atomic nuclei  ×10^{17} 2.3  Does not depend strongly on size of nucleus^{[20]} 
Neutron star  ×10^{18} 1  
Stellarmass black hole  ×10^{18} 1 
Density of a 4solarmass black hole Event horizon radius is 12 km. 
Common units
The SI unit for density is:
 kilograms per cubic meter (kg/m^{3})
Litres and metric tons are not part of the SI, but are acceptable for use with it, leading to the following units:
 kilograms per liter (kg/L)
 grams per milliliter (g/mL)
 metric tons per cubic meter (t/m^{3})
Densities using the following metric units all have exactly the same numerical value, one thousandth of the value in (kg/m^{3}). Liquid water has a density of about 1 kg/dm^{3}, making any of these SI units numerically convenient to use as most solids and liquids have densities between 0.1 and 20 kg/dm^{3}.
 kilograms per cubic decimetre (kg/dm^{3})

grams per cubic centimetre (g/cm^{3})
 1 gram/cm^{3} = 1000 kg/m^{3}
 megagrams (metric tons) per cubic metre (Mg/m^{3})
In US customary units density can be stated in:
 Avoirdupois ounces per cubic inch (oz/cu in)
 Avoirdupois pounds per cubic inch (lb/cu in)
 pounds per cubic foot (lb/cu ft)
 pounds per cubic yard (lb/cu yd)
 pounds per US liquid gallon (lb/gal)
 pounds per US bushel (lb/bu)
 slugs per cubic foot
Imperial units differing from the above (as the Imperial gallon and bushel differ from the US units) in practice are rarely used, though found in older documents. The density of precious metals could conceivably be based on Troy ounces and pounds, a possible cause of confusion.
See also
 List of elements by density
 Air density
 Area density
 Bulk density
 Buoyancy
 Charge density
 Density prediction by the Girolami method
 Dord
 Energy density
 Lighter than air
 Linear density
 Number density
 Orders of magnitude (density)
 Orthobaric density
 Paper density
 Specific weight
 Spice (oceanography)
 Standard temperature and pressure
References
 ^
 ^
 ^ Archimedes, A Gold Thief and Buoyancy – by Larry "Harris" Taylor, Ph.D.
 ^ Vitruvius on Architecture, Book IX, paragraphs 9–12, translated into English and in the original Latin.
 ^
 ^ Fact or Fiction?: Archimedes Coined the Term "Eureka!" in the Bath, Scientific American, December 2006.
 ^ New carbon nanotube struructure aerographite is lightest material champ. Phys.org (July 13, 2012). Retrieved on July 14, 2012.
 ^ Aerographit: Leichtestes Material der Welt entwickelt – SPIEGEL ONLINE. Spiegel.de (July 11, 2012). Retrieved on July 14, 2012.
 ^ ^{a} ^{b}
 ^ ^{a} ^{b}
 ^
 ^
 ^ CRC Press Handbook of tables for Applied Engineering Science, 2nd Edition, 1976, Table 159
 ^ glycerol composition at. Physics.nist.gov. Retrieved on July 14, 2012.
 ^ ^{a} ^{b} Hugh D. Young; Roger A. Freedman. University Physics with Modern Physics. AddisonWesley; 2012. ISBN 9780321696861. p. 374.
 ^
 ^
 ^
 ^ Extreme Stars: White Dwarfs & Neutron Stars, Jennifer Johnson, lecture notes, Astronomy 162, Ohio State University. Accessed: May 3, 2007.
 ^ Nuclear Size and Density, HyperPhysics, Georgia State University. Accessed: June 26, 2009.
External links
 Video: Density Experiment with Oil and Alcohol
 Video: Density Experiment with Whiskey and Water
 Glass Density Calculation – Calculation of the density of glass at room temperature and of glass melts at 1000 – 1400°C
 List of Elements of the Periodic Table – Sorted by Density
 Calculation of saturated liquid densities for some components
 Field density test
 Online calculator for densities and partial molar volumes of aqueous solutions of some common electrolytes and their mixtures, at temperatures up to 323.15 K.
 Water – Density and specific weight
 Temperature dependence of the density of water – Conversions of density units
 A delicious density experiment
 Water density calculator Water density for a given salinity and temperature.
 Liquid density calculator Select a liquid from the list and calculate density as a function of temperature.
 Gas density calculator Calculate density of a gas for as a function of temperature and pressure.
 Densities of various materials.
 Determination of Density of Solid, instructions for performing classroom experiment.
 density prediction
 density prediction