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Density
Density
Published: 2013/02/28
Channel: Mark Drollinger
Density Practice Problems
Density Practice Problems
Published: 2012/08/25
Channel: Tyler DeWitt
5 Facts about Density
5 Facts about Density
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Density - Why does oil float on water?
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Channel: Smart Learning for All
The Sci Guys: Science at Home - SE1 - EP5: 12 Layer Liquid Density Tower
The Sci Guys: Science at Home - SE1 - EP5: 12 Layer Liquid Density Tower
Published: 2013/03/07
Channel: The Sci Guys
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What is density?
Published: 2013/09/09
Channel: The Science Classroom
Density - Why do firemen crawl in smoke filled rooms?
Density - Why do firemen crawl in smoke filled rooms?
Published: 2017/08/19
Channel: Smart Learning for All
Density
Density
Published: 2014/06/04
Channel: Bozeman Science
Denser Than You Think - Science Experiment
Denser Than You Think - Science Experiment
Published: 2015/11/17
Channel: DaveHax
The Density of a Black Hole - How The Universe Works
The Density of a Black Hole - How The Universe Works
Published: 2010/06/23
Channel: Discovery UK
Awesome Sugar Water Density - Science Experiment
Awesome Sugar Water Density - Science Experiment
Published: 2015/12/14
Channel: CrazyRussianHacker
Physics - What is Density?
Physics - What is Density?
Published: 2017/07/20
Channel: Don't Memorise
The Spangler Effect - Density Science Season 02 Episodes 05 and 06
The Spangler Effect - Density Science Season 02 Episodes 05 and 06
Published: 2013/04/04
Channel: TheSpanglerEffect
Amazing 9 Layer Density Tower - Sick Science! #012
Amazing 9 Layer Density Tower - Sick Science! #012
Published: 2010/12/08
Channel: Sick Science!
How To: Find Density/Mass/Volume (EASY equation w/ practice problems)
How To: Find Density/Mass/Volume (EASY equation w/ practice problems)
Published: 2015/02/26
Channel: ScienceMade Easier
What is Density?
What is Density?
Published: 2015/04/28
Channel: ScienceMadeEasy
Density: A Story of Archimedes and the Gold Crown
Density: A Story of Archimedes and the Gold Crown
Published: 2012/08/28
Channel: Tyler DeWitt
Evaluate: Mass, Volume, and Density
Evaluate: Mass, Volume, and Density
Published: 2013/12/20
Channel: Science Bits
Thinking about sinking and floating: Density
Thinking about sinking and floating: Density
Published: 2013/09/12
Channel: Caitlin Webb
What is Density? How to Calculate Density
What is Density? How to Calculate Density
Published: 2015/09/10
Channel: The Science Classroom
Mass vs. Density
Mass vs. Density
Published: 2012/12/20
Channel: FlinnScientific
Gravity is Just Density
Gravity is Just Density
Published: 2017/02/16
Channel: Eric Dubay
Eggs & Salt Water - Water Density Science Experiment
Eggs & Salt Water - Water Density Science Experiment
Published: 2013/05/09
Channel: CrazyRussianHacker
Density Experiment Float or Sink
Density Experiment Float or Sink
Published: 2016/01/02
Channel: CrazyRussianHacker
Float or Sink - Cool Science Experiment
Float or Sink - Cool Science Experiment
Published: 2011/01/12
Channel: SpanglerScienceTV
How to Calculate Density of Liquids - With Examples
How to Calculate Density of Liquids - With Examples
Published: 2016/07/25
Channel: cleanairfilms
The Density Song --  June 28, 2012 Mr. Edmonds
The Density Song -- June 28, 2012 Mr. Edmonds
Published: 2012/06/28
Channel: dsecms
Determination of Density of Solid - MeitY OLabs
Determination of Density of Solid - MeitY OLabs
Published: 2017/02/03
Channel: amritacreate
Density of Matter (Hindi)-पदार्थ घनत्व
Density of Matter (Hindi)-पदार्थ घनत्व
Published: 2016/08/01
Channel: Pratham Open School
Density - Explained
Density - Explained
Published: 2015/05/21
Channel: Chem Academy
Density to Molarity Conversion : Chem Class
Density to Molarity Conversion : Chem Class
Published: 2014/03/14
Channel: eHowEducation
speed density remap - I messed up!
speed density remap - I messed up!
Published: 2017/08/15
Channel: Jimmy Oakes
The Seven Densities of Evolution - Bentinho Massaro LIVE (4.6.15)
The Seven Densities of Evolution - Bentinho Massaro LIVE (4.6.15)
Published: 2015/04/07
Channel: Bentinho Massaro
SI Units and Density
SI Units and Density
Published: 2014/11/05
Channel: Teacher's Pet
What is density?
What is density?
Published: 2016/01/12
Channel: Richard Louie Chemistry Lectures
DENSITY TOWER Easy Kids Science Experiments
DENSITY TOWER Easy Kids Science Experiments
Published: 2012/12/17
Channel: WhizKidScience
Eureka! Episode 25 - Volume and Density
Eureka! Episode 25 - Volume and Density
Published: 2012/01/29
Channel: Webquestify
Master Density Class 101- Rob Gauthier ( densities and dimensions)
Master Density Class 101- Rob Gauthier ( densities and dimensions)
Published: 2013/09/02
Channel: The Official ET Whisperer Channel - Rob Gauthier
MICHAEL WOLF: Hong Kong
MICHAEL WOLF: Hong Kong's Architecture of Density
Published: 2016/10/10
Channel: Yitiao Video 一条视频
How to Find the Density of a Solid or Liquid
How to Find the Density of a Solid or Liquid
Published: 2014/05/05
Channel: science4schools
Buoyancy and Density
Buoyancy and Density
Published: 2008/01/07
Channel: ScienceOnline
7 Layer Density - Cool Science Experiment
7 Layer Density - Cool Science Experiment
Published: 2010/02/16
Channel: Sick Science!
Performing Proper Density Analysis Part 1
Performing Proper Density Analysis Part 1
Published: 2009/12/04
Channel: esri
Measuring Density of Small Irregular Objects
Measuring Density of Small Irregular Objects
Published: 2016/08/07
Channel: Cody'sLab
Determination of Density of Solid  - MeitY OLabs
Determination of Density of Solid - MeitY OLabs
Published: 2015/01/31
Channel: amritacreate
Density dependence and density independence
Density dependence and density independence
Published: 2014/04/11
Channel: Burkebiology
Density
Density
Published: 2016/10/19
Channel: airwindows
Find the Density of a Gas (Ideal Gas Law)
Find the Density of a Gas (Ideal Gas Law)
Published: 2013/03/13
Channel: chemistNATE
Determining Volume from Density and Mass
Determining Volume from Density and Mass
Published: 2012/08/15
Channel: kentchemistry.com
Linear and Planar Densities for Face Centered Cubic (FCC) Unit Cells
Linear and Planar Densities for Face Centered Cubic (FCC) Unit Cells
Published: 2014/10/20
Channel: NUS Chem Emelyn Tan
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WIKIPEDIA ARTICLE

From Wikipedia, the free encyclopedia
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Density
Common symbols
ρ
D
SI unit kg/m3
SI dimension L^{-3}M
A graduated cylinder containing various coloured liquids with different densities.

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]

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.

History

In a well-known 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/m3) and the cgs unit of gram per cubic centimetre (g/cm3) are probably the most commonly used units for density.1,000 kg/m3 equals 1 g/cm3. (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() = dm/dV, where dV is an elementary volume at position r. The mass of the body then can be expressed as

Non-compact 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 close-packing of equal spheres the non-void 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 non-compact 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

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 :

where is the density at a reference temperature, is the thermal expansion coefficient of the material at temperatures close to .

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 ρi in a solution sums to density of the solution.

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:

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.

Densities

Molar volumes of liquid and solid phase of elements

Molar volumes of liquid and solid phase of elements

Water

Density of liquid water at 1 atm pressure
Temp. (°C)[note 1] Density (kg/m3)
−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:
  1. ^ Values below 0 °C refer to supercooled water.

Air

Air density vs. temperature
Density of air at 1 atm pressure
T (°C) ρ (kg/m3)
−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

Densities of various materials covering a range of values
Material ρ (kg/m3)[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]
Ice 916.7 At temperature < 0 °C
Sodium 970
Water (fresh) 1,000 At 4 °C, the temperature of its maximum density
Water (salt) 1,030
Liquid oxygen 1,141 At approx. −219 °C
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
Rhenium 21,020
Platinum 21,450
Iridium 22,420
Osmium 22,570
Notes:
  1. ^ Unless otherwise noted, all densities given are at standard conditions for temperature and pressure, that is, 273.15 K (0.00 °C) and 100 kPa (0.987 atm).
  2. ^ a b c Air contained in material excluded when calculating density

Others

Entity ρ (kg/m3) Notes
Interstellar medium 1×10−19 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]
Super-massive black hole 9×105 Density of a 4.5-million-solar-mass black hole
Event horizon radius is 13.5 million km.
White dwarf star 2.1×109 Approx.[19]
Atomic nuclei 2.3×1017 Does not depend strongly on size of nucleus[20]
Neutron star 1×1018
Stellar-mass black hole 1×1018 Density of a 4-solar-mass black hole
Event horizon radius is 12 km.

Common units

The SI unit for density is:

Litres and metric tons are not part of the SI, but are acceptable for use with it, leading to the following units:

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 kg/dm3, making any of these SI units numerically convenient to use as most solids and liquids have densities between 0.1 and 20 kg/dm3.

  • kilograms per cubic decimetre (kg/dm3)
  • grams per cubic centimetre (g/cm3)
    • 1 gram/cm3 = 1000 kg/m3
  • megagrams (metric tons) per cubic metre (Mg/m3)

In US customary units density can be stated in:

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 Imperial gallon was based on the concept that an Imperial fluid ounce of water would have a mass of one Avoirdupois ounce, and indeed 1 g/cc ≈ 1.00224129 ounces per Imperial fluid ounce = 10.0224129 lbs per Imperial gallon. The density of precious metals could conceivably be based on Troy ounces and pounds, a possible cause of confusion.

See also

References

  1. ^ The National Aeronautic and Atmospheric Administration's Glenn Research Center. "Gas Density Glenn research Center". grc.nasa.gov. 
  2. ^ "Density definition in Oil Gas Glossary". Oilgasglossary.com. Archived from the original on August 5, 2010. Retrieved September 14, 2010. 
  3. ^ Archimedes, A Gold Thief and Buoyancy – by Larry "Harris" Taylor, Ph.D.
  4. ^ Vitruvius on Architecture, Book IX, paragraphs 9–12, translated into English and in the original Latin.
  5. ^ "EXHIBIT: The First Eureka Moment". Science. 305 (5688): 1219e. 2004. doi:10.1126/science.305.5688.1219e. 
  6. ^ Fact or Fiction?: Archimedes Coined the Term "Eureka!" in the Bath, Scientific American, December 2006.
  7. ^ New carbon nanotube struructure aerographite is lightest material champ. Phys.org (July 13, 2012). Retrieved on July 14, 2012.
  8. ^ Aerographit: Leichtestes Material der Welt entwickelt – SPIEGEL ONLINE. Spiegel.de (July 11, 2012). Retrieved on July 14, 2012.
  9. ^ a b "Re: which is more bouyant [sic] styrofoam or cork". Madsci.org. Retrieved September 14, 2010. 
  10. ^ a b Raymond Serway; John Jewett (2005), Principles of Physics: A Calculus-Based Text, Cengage Learning, p. 467, ISBN 0-534-49143-X 
  11. ^ "Wood Densities". www.engineeringtoolbox.com. Retrieved October 15, 2012. 
  12. ^ "Density of Wood". www.simetric.co.uk. Retrieved October 15, 2012. 
  13. ^ CRC Press Handbook of tables for Applied Engineering Science, 2nd Edition, 1976, Table 1-59
  14. ^ glycerol composition at. Physics.nist.gov. Retrieved on July 14, 2012.
  15. ^ a b Hugh D. Young; Roger A. Freedman. University Physics with Modern Physics. Addison-Wesley; 2012. ISBN 978-0-321-69686-1. p. 374.
  16. ^ Density of the Earth, wolframalpha.com 
  17. ^ Density of Earth's core, wolframalpha.com 
  18. ^ Density of the Sun's core, wolframalpha.com 
  19. ^ Extreme Stars: White Dwarfs & Neutron Stars, Jennifer Johnson, lecture notes, Astronomy 162, Ohio State University. Accessed: May 3, 2007.
  20. ^ Nuclear Size and Density, HyperPhysics, Georgia State University. Accessed: June 26, 2009.

External links

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