# What is Density: Definition and 1000 Discussions

The density (more precisely, the volumetric mass density; also known as specific mass), 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:

ρ
=

m
V

{\displaystyle \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, 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.
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 relative to water 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.

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2. ### How can I calculate the Density and Volume of a mechanical mixture (atoms)?

Is it correct that: density = [ 0.09 * (density of NaCl) ] + [0.91 * (density of water) ] volume = [ (volume of water) + ( { (volume of NaCl) / 48} * 9) ] Thank you so much for any advice.
3. ### I Black Holes in Curved Spacetime

We've been talking in another thread about supermassive black holes. That has me thinking about really, really big BH's - so large that the spacetime curvature and evolution of the universe matters. Let's start by defining the density of a black hole as its mass divided by the volume enclosed...
4. ### Confusion on product rule for mass of differential volume element

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5. ### Density calculation sometimes can be confusing

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6. ### Varying fluid (density) in a cylinder rolling along an inclined plane

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7. ### Calculate the bouyancy of two spheres joined by a rope submerged in seawater

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8. ### Density of Flow Along a Tube Under the Action of Advancing Piston

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9. ### Deriving density formulae from first principles

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11. ### Integrate disk mass density

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13. ### Density of a patch of an accretion disk

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14. ### Density of a sphere that has a cavity

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23. ### I Density parameter and curvature index

For Ω=1, κ=0. Does the value of κ simply follow from the value of Ω, or can its value have an independent existence? So if Ω>1, does κ have to be 1?
24. ### I Derivation of two-electron density operator

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25. ### Energy Density with a Dielectric

I am confused about how the electric field changes in this problem - is E' = E/Ke=E/2? Is E = V/d a correct usage? When I solve it this way, the answer is incorrect: change in energy density = (1/2)ε(E'2- E2) = (1/2)ε(E2/4 - E2) = (1/2)ε(-3/4)(V/2d)2. What am I doing wrong? Thanks.
26. ### Finding Charge Density on the Surface of a Slab

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27. ### Magnitude of the Line Charge Density of a Power Line

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28. ### I The electron as a smeared charge density

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39. ### Probability density of a 1-D Tonk Gas

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40. ### Why does less dense air rise and more dense air come down?

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41. ### I Deriving expression for resistance in terms of current density

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42. ### Density of cylinder that undergoes vertical oscillation

I'm not sure where to start...
43. ### 2 Capacitors and energy density

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44. ### A Mixture density neural network prediction bias

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45. ### Calculation of Electrostatic Potential Given a Volume Charge Density

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46. ### Solve this problem that involves a probability density function

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47. ### A Lagrangian density for the spinor fields

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48. ### Problem regarding vapour density of a mixture

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49. ### Surface density of stars in a Galaxy

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50. ### Flux density help please (seawater flowing in a tube)

What is the flux density of salt in a horizontal tube 10 cm in length connecting seawater (salinity = 30 g/l) to a tank of freshwater (salinity ~ 0) assuming no advection occurs?