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{{about|mass density}}
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The '''mass density''' or '''density''' of a material is defined as its [[mass]] per unit [[volume]]. The symbol most often used for density is ρ (the Greek letter [[Rho (letter)|rho]]). In some cases (for instance, in the United States oil and gas industry), density is also defined as its [[weight]] per unit [[volume]];<ref>{{cite web|url=http://oilgasglossary.com/density.html |title=Density definition in Oil Gas Glossary |publisher=Oilgasglossary.com |date= |accessdate=2010-09-14}}</ref> although, this quantity is more properly called [[specific weight]]. Different materials usually have different densities, so density is an important concept regarding [[buoyancy]], purity and [[packaging]]. [[Osmium]] is the densest known substance at [[standard conditions for temperature

Less dense fluids float on more dense fluids if they do not mix. This concept can be extended, with some care, to less dense solids floating on more dense fluids. If the average density (including any air below the waterline) of an object is less than water (1.0 g per mL) it will float in water and if it is more than water's it will sink in water.

In some cases density is expressed as the [[dimensionless]] quantities [[specific gravity]] (SG) or [[relative density]] (RD), in which case it is expressed in multiples of the density of some other standard material, usually water or air/gas. (For example, a specific gravity less than one means that the substance floats in water.)

The mass density of a material varies with temperature and pressure. (The variance is typically small for solids and liquids and much greater for gasses.) Increasing the pressure on an object decreases the volume of the object and therefore increase its density. Increasing the temperature of a substance (with some exceptions) decreases its density by increasing the volume of that substance. In most materials, heating the bottom of a fluid results in [[convection]] of the heat from bottom to top of the fluid due to the decrease of 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 called its [[specific volume]], a representation commonly 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.

==History==
In a well-known but probably apocryphal tale, [[Archimedes]] was given the task of determining whether [[Hiero II of Syracuse|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]].<ref>[http://www-personal.umich.edu/~lpt/archimedes.htm Archimedes, A Gold Thief and Buoyancy] - by Larry "Harris" Taylor, Ph.D.</ref> 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 took a relaxing 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 (fluid)|displacement]] of the water. Upon this discovery, he leaped from his bath and went running naked through the streets shouting, "Eureka! Eureka!" (Εύρηκα! Greek "I found it"). As a result, the term "[[Eureka (word)|eureka]]" entered common parlance and is used today to indicate a moment of enlightenment.

The story first appeared in written form in [[Vitruvius]]' [[De architectura|books of architecture]], two centuries after it supposedly took place.<ref>[http://penelope.uchicago.edu/Thayer/E/Roman/Texts/Vitruvius/9*.html Vitruvius on Architecture, Book IX], paragraphs 9-12, translated into English and [http://penelope.uchicago.edu/Thayer/L/Roman/Texts/Vitruvius/9*.html in the original Latin].</ref> 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.<ref>[http://www.sciencemag.org/cgi/content/summary/305/5688/1219e The first Eureka moment], ''Science'' '''305''': 1219, August 2004.</ref><ref>[http://www.sciam.com/article.cfm?articleID=5F1935E9-E7F2-99DF-3F1D1235AF1D2CD1 Fact or Fiction?: Archimedes Coined the Term "Eureka!" in the Bath], ''Scientific American'', December 2006.</ref>

Mathematically, density is defined as mass divided by volume:

:<math> \rho = \frac{m}{V},</math>

where {{math|ρ}} is the density, {{math|m}} is the mass, and {{math|V}} is the volume. From this equation, mass density must have units of a unit of mass per unit of 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]] ({{math|kg/m³}}) and the [[cgs]] unit of [[gram]] per [[cubic centimetre]] ({{math|g/cm³}}) are probably the most common used units for density. (The cubic centimeter can be alternately called a ''millilitre'' or a ''cc''.) One {{math|g/cm³}} equals {{math|1000 kg/m³}}. 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. Further, density may be expressed in terms of weight density (the weight of the material per unit volume) or as a ratio of the density with the density of a common material such as air or water.

==Measurement of density==
The density at any point of a [[Homogeneous (chemistry)|homogeneous]] object equals its total mass divided by its total volume. The mass is normally measured with an appropriate [[weighing scale|scale or balance]]; the volume may be measured directly (from the geometry of the object) or by the displacement of a fluid. [[Hydrostatic weighing]], for instance uses, the displacement of water due to a submerged object to determine the density of the object.

If the body is not homogeneous, then the density is a function of the position. 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: {{math|ρ('''r''')}}={{math|dm/dV}}, where {{math|dV}} is an elementary volume at position {{math|r}}. The mass of the body then can be expressed as

:<math>
m = \int_V \rho(\mathbf{r})\,dV.
</math>

The density of granular material can be ambiguous, depending on exactly how its volume is defined, and this may cause confusion in measurement. A common example is sand: if it is gently poured into a container, the density will be low; if the same sand is then compacted, it will occupy less volume and consequently exhibit a greater density. This is because sand, like all powders and granular solids, contains a lot of air space in between individual grains. The density of the material including the air spaces is the [[bulk density]], which differs significantly from the density of an individual grain of sand with no air included.

==Changes of density==
{{main|Compressibility|Thermal expansivity}}

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&nbsp;°C and 4&nbsp;°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<sup>−6</sup>&nbsp;[[bar (unit)|bar]]<sup>−1</sup> (1&nbsp;bar=0.1&nbsp;MPa) and a typical [[thermal expansivity]] is 10<sup>−5</sup>&nbsp;[[Kelvin|K]]<sup>−1</sup>. 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

:<math>
\rho = \frac {MP}{RT}, \,
</math>

where {{math|M}} is the [[molar mass]], {{math|P}} is the pressure, {{math|R}} is the [[Gas constant|universal gas constant]], and {{math|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.

==Density of water (at 1 atm)==
{{See also|Water (molecule)#Density of water and ice|l1=Water density}}

{| class="wikitable"
|-
! Temp (°C) !! Density (kg/m<sup>3</sup>)
|-
|100||958.4
|-
|80||971.8
|-
|60||983.2
|-
|40||992.2
|-
|30||995.6502
|-
|25||997.0479
|-
|22||997.7735
|-
|20||998.2071
|-
|15||999.1026
|-
|10||999.7026
|-
|4||999.9720
|-
|0||999.8395
|-
|−10||998.117
|-
|−20||993.547
|-
|−30||983.854
|-
|colspan="2"| <small>The density of water in kilograms per cubic metre (SI unit)<br> at various temperatures in degrees Celsius.<br>The values below 0 °C refer to [[supercooling|supercooled]] water.
|}

==Density of air (at 1 atm)==
{{Main|Density of air}}
[[Image:MyChart.jpg|thumb|right|400px|Density ''vs.'' Temperature]]
{| class="wikitable" style="text-align:center; float:left;"
|-
!''T'' in [[Celsius|°C]] !! ''ρ'' in kg/m<sup>3</sup>
|-
| –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
|}
{{-}}

==Density of solutions==
The density of a solution is the sum of [[mass concentration (chemistry)|mass (massic) concentrations]] of the components of that solution.<br>
Mass (massic) concentration of a given component ρ<sub>i</sub> in a solution can be called partial density of that component.
:<math>\rho = \sum_i \rho_i \,</math>

==Densities of various materials==
{{See|Orders of magnitude (density)}}
{| class="wikitable sortable" style="text-align:center; float:left;"
|-
! Material !! ''ρ'' in kg/m<sup>3</sup> !! Notes
|-
| [[Interstellar medium]] || 10<sup>−25</sup> − 10<sup>−15</sup> || Assuming 90% H, 10% He; variable T
|-
| [[Earth's atmosphere]] || 1.2 || At sea level
|-
| [[Aerogel]] || 1 − 2 ||
|-
| [[Styrofoam]] || 30 − 120<ref name="madsci1">{{cite web|url=http://www.madsci.org/posts/archives/mar2000/954534602.Ph.r.html |title=Re: which is more {{sic|bou|yant|nolink=y}} styrofoam or cork |publisher=Madsci.org |date= |accessdate=2010-09-14}}</ref> ||
|-
| [[Cork (material)|Cork]] || 220 − 260<ref name="madsci1"/> ||
|-
| [[Ice]] || 916.7 <!-- Sourced from the "Ice" page -->
|-
| [[Water]] (fresh) || 1000 || At [[Standard conditions for temperature and pressure|STP]]
|-
| [[Water]] (salt) || 1030 ||
|-
| [[Plastics]] || 850 − 1400 || For [[polypropylene]] and [[PETE]]/[[PVC]]
|-
| [[Glycerol]]<ref>[http://physics.nist.gov/cgi-bin/Star/compos.pl?matno=174 glycerol composition at physics.nist.gov]</ref><ref>[http://wiki.answers.com/Q/Density_of_glycerin Glycerol density at answers.com]</ref> || 1261 ||
|-
| [[Aluminium]] || 2700 || Near room temperature
|-
| The [[Earth]] || 5515.3 || Mean density
|-
| [[Iron]] || 7874 || Near [[room temperature]]
|-
| [[Copper]] || 8920 − 8960 || Near room temperature
|-
| [[Silver]] || 10490 || Near room temperature
|-
| [[Lead]] || 11340 || Near room temperature
|-
| The [[Inner Core]] of the Earth || ~13000 || As listed in [[Earth]]
|-
| [[Uranium]] || 19100 || Near room temperature
|-
| [[Tungsten]] || 19250 || Near room temperature
|-
| [[Gold]] || 19300 || Near room temperature
|-
| [[Platinum]] || 21450 || Near room temperature
|-
| [[Iridium]] || 22500 || Near room temperature
|-
| [[Osmium]] || 22610 || Near room temperature
|-
| The core of the [[Sun]] || ~150000 ||
|-
| [[White dwarf]] star || 1 × 10<sup>9</sup><ref name="osln">[http://www.astronomy.ohio-state.edu/~jaj/Ast162/lectures/notesWL22.pdf Extreme Stars: White Dwarfs & Neutron Stars], Jennifer Johnson, lecture notes, Astronomy 162, [[Ohio State University]]. Accessed on line May 3, 2007.</ref> ||
|-
| [[Atomic nuclei]] || 2.3 × 10<sup>17</sup>&nbsp;<ref>[http://hyperphysics.phy-astr.gsu.edu/HBASE/Nuclear/nucuni.html Nuclear Size and Density], HyperPhysics, Georgia State University. Accessed on line June 26, 2009.</ref> || Does not depend strongly on size of nucleus
|-
| [[Neutron star]] || 8.4 × 10<sup>16</sup> − 1 × 10<sup>18</sup> ||
|-
| [[Black hole]] || 4 × 10<sup>17</sup> || Mean density inside the [[Schwarzschild radius]] of an Earth-mass [[black hole]] (theoretical)
|}
{{-}}

==Density of composite material==
In the United States, ASTM specification D792-00<ref>(2004). ''Test Methods for Density and Specific Gravity (Relative Density) of Plastics by Displacement''. ASTM Standard D792-00. Vol 81.01. American Society for Testing and Materials. West Conshohocken. PA.</ref> describes the steps to measure the density of a composite material.
:<math>
\rho = \frac{W_a}{W_a + W_w - W_b} \left (0.9975 \right ) \,
</math>

where:
:<math>\rho</math> is the density of the composite material, in g/cm<sup>3</sup>

and
:<math>W_a</math> is the weight of the specimen when hung in the air
:<math>W_w</math> is the weight of the partly immersed wire holding the specimen
:<math>W_b</math> is the weight of the specimen when immersed fully in distilled water, along with the partly immersed wire holding the specimen
:<math>0.9975</math> is the density in g/cm<sup>3</sup> of the distilled water at 23 °C.

==Other common units==

The [[SI]] unit for density is:
* [[kilogram]]s per [[cubic metre]] (kg/m³)

Litres and metric tons are not part of the SI, but are acceptable for use with it, leading to the following units:
* [[kilogram]]s per [[litre]] (kg/L)
* [[gram]]s per [[millilitre]] (g/mL)
* [[metric ton]]s per cubic metre (t/m³)

Densities using the following metric units all have exactly the same numerical value, one thousandth of the value in (kg/m³). Liquid [[water]] has a density of about 1&nbsp;kg/dm³, making any of these SI units numerically convenient to use as most [[solid]]s and [[liquid]]s have densities between 0.1 and 20&nbsp;kg/dm³.
* kilograms per cubic decimetre (kg/dm³)
* grams per cubic centimetre (g/cc, gm/cc or g/cm³)
* megagrams per cubic metre (Mg/m³)

In [[US customary units|U.S. customary units]] density can be stated in:
* [[Avoirdupois ounce]]s per [[cubic inch]] (oz/cu&nbsp;in)
* [[Pound (mass)|Avoirdupois pounds]] per cubic inch (lb/cu&nbsp;in)
* pounds per [[cubic foot]] (lb/cu&nbsp;ft)
* pounds per [[cubic yard]] (lb/cu&nbsp;yd)
* pounds per [[U.S. liquid gallon]] (lb/gal)
* pounds per U.S. [[bushel]] (lb/bu)
* [[slug (mass)|slugs]] per cubic foot.
In principle there are [[Imperial units]] different from the above as the Imperial gallon and bushel differ from the U.S. units, but in practice they are no longer used, though found in older documents. The density of [[precious metal]]s could conceivably be based on [[Troy weight|Troy]] ounces and pounds, a possible cause of confusion.

==See also==
<div style="-moz-column-count:3; column-count:3;">
* [[List of elements by density]]
* [[Charge density]]
* [[Buoyancy]]
* [[Bulk density]]
* [[Dord]]
* [[Energy density]]
* [[Lighter than air]]
* [[Number density]]
* [[Orthobaric density]]
* [[Specific weight]]
* [[Spice (oceanography)]]
* [[Standard temperature and pressure]]
* [[Orders of magnitude (density)]]
* [[Girolami method|Density prediction by the Girolami method]]
</div>

==References==
{{Reflist}}

==External links==
* [http://glassproperties.com/density/room-temperature/ Glass Density Calculation - Calculation of the density of glass at room temperature and of glass melts at 1000 - 1400°C]
* [http://www.science.co.il/PTelements.asp?s=Density List of Elements of the Periodic Table - Sorted by Density]
* [http://ddbonline.ddbst.de/DIPPR105DensityCalculation/DIPPR105CalculationCGI.exe Calculation of saturated liquid densities for some components]
* [http://www.denichsoiltest.com/field/field-density-test.html field density test]
* [http://www.aim.env.uea.ac.uk/aim/density/density_eletrolyte.php On-line calculator for densities and partial molar volumes of aqueous solutions of some common electrolytes and their mixtures, at temperatures up to 323.15 K.]
* [http://www.engineeringtoolbox.com/water-density-specific-weight-d_595.html Water - Density and Specific Weight]
* [http://www.sengpielaudio.com/ConvDensi.htm Temperature dependence of the density of water - Conversions of density units]
* [http://www.adamequipment.com/education/Documents/EdExp1.pdf A delicious density experiment]
* [http://www.enggcyclopedia.com/welcome-to-enggcyclopedia/calculators/liquid-density Liquid density calculator] Select a liquid from the list and calculate density as a function of temperature.
* [http://www.enggcyclopedia.com/welcome-to-enggcyclopedia/thermodynamics/gas-density Gas density calculator] Calculate density of a gas for as a function of temperature and pressure.

[[Category:Density| ]]
[[Category:Continuum mechanics]]
[[Category:Fundamental physics concepts]]
[[Category:Introductory physics]]
[[Category:Physical quantities]]
[[Category:Physical chemistry]]
[[Category:Basic meteorological concepts and phenomena]]
[[Category:State functions]]

[[af:Digtheid]]
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[[ast:Densidá (física)]]
[[bn:ঘনত্ব]]
[[be:Шчыльнасць]]
[[bg:Плътност]]
[[ca:Densitat]]
[[cs:Hustota]]
[[cy:Dwysedd]]
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[[fr:Masse volumique]]
[[fy:Tichtheid]]
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[[io:Specifika pezo]]
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[[is:Eðlismassi]]
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[[he:צפיפות החומר]]
[[ka:სიმკვრივე]]
[[sw:Densiti]]
[[ht:Dansite]]
[[la:Densitas et Spissitudo]]
[[lv:Blīvums]]
[[lb:Dicht]]
[[lt:Tankis]]
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[[hu:Sűrűség]]
[[mk:Густина]]
[[ml:സാന്ദ്രത]]
[[mr:घनता]]
[[ms:Ketumpatan]]
[[mn:Нягт]]
[[nl:Dichtheid (natuurkunde)]]
[[ja:密度]]
[[no:Tetthet]]
[[nn:Tettleik]]
[[nds:Dicht]]
[[pl:Gęstość]]
[[pt:Densidade]]
[[ro:Densitate]]
[[ru:Плотность]]
[[sq:Dendësia]]
[[simple:Density]]
[[sl:Gostota]]
[[szl:Gynstość]]
[[ckb:چڕی]]
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[[sv:Densitet]]
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[[te:సాంద్రత]]
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[[uk:Густина]]
[[ur:کثافت]]
[[vi:Khối lượng riêng]]
[[yo:Kíkisí]]
[[zh:密度]]

Revision as of 14:33, 10 January 2011

I'M SO HORNY. S UCK MY BIG KNOB. arsearsearse

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