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Water is normally assumed to be an incompressible fluid - for example in the context of calculations involving water pressure.

I wondered whether that is strictly true, or an approximation? Later I noticed some side note implying water is not fully incompressible.

Of course that makes sense, as there are not many things "perfect" in nature in this sense, like maybe supraconductivity and suprafluidity.


Now, why, or "in what kind of way", is water only almost incompressible - is it caused by impurities like gases and other fluids in the water?

Or is it not completely incompressible in some fundamental way?

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I'm interested in this too. The inventor George Constantinescu supposedly held the view that water was compressible and demonstrated an open artillery shell-casing full of water was able to "bounce" a closely-fitting weight that ran on a vertical rail. –  PlaysDice Apr 24 at 17:21
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Applying not just to water but more generally, nothing can be completely incompressible. If it were, then part of the material would have to instantly respond to a force acting on a different part. This would violate FTL. –  RandomUser Apr 24 at 21:46
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@RandomUser FTL? –  Bernhard Apr 26 at 5:47
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@Bernhard Faster than light. A perfectly incompressible material would be able to transfer force instantaneously, and hence faster than light. This can not be. –  RandomUser Apr 26 at 7:31

2 Answers 2

up vote 7 down vote accepted

Formally, the incompressibility of a fluid is defined by the compressibility, $$ \beta=\frac1\rho\,\frac{\partial\rho}{\partial p} $$ where $\rho$ is the mass density and $p$ the gas pressure. This means that, the compressibility is the measure of how much the density (volume) changes when a pressure is applied.

For water at standard pressure, this works out to be on the order $10^{-10}\,m^2/N$ which is pretty darn small but definitely non-zero. This value isn't due to impurities in the water, it is due to the properties of H$_2$0 itself. If you look at the $\rho\,{\rm vs}\,p$ plot, you can see how the density changes with both (solid green line):

pressure-density diagram for water

The left-most end of the chart is standard pressure of 1 atm (roughly 0.1 MPa). As you can see, it is not until very high pressures that the density begins to really change. But at most every-day temperatures the deviations from horizontal are negligible.

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Matter is made up from point like fundamental particles, like electrons and quarks, that have zero volume. This puts us in the interesting position where the true volume of all matter is zero, and the only reason that everything doesn't instantly collapse into a point of zero volume is that the pointlike fundamental particles maintain a finite distance from each other due to a variety of forces. For example in a hydrogen atom the uncertainty principle restricts how close together the electron and proton can get. You can compress a hydrogen atom, but it costs energy so there will be a repulsive force resisting the compression. Similarly if you try and squeeze two hydrogen atoms together the exchange force resists you.

The point of this rather abstruse discussion is that for a system to be incompressible the force between pairs of fundamental particles would have to become infinite. For any finite force the particles can be squeezed together, and therefore the system can be compressed. No such infinite forces are known in nature, and therefore there is no such thing as a fundamentally incompressible system.

The compressibility of macroscopic systems like water is the reciprocal of the bulk modulus, which for water is about 2.2 GPa. This is high compared to readily compressible systems like gases, but low compared to steel at 160 Gpa and diamond at 443 GPa. So compared to steel and diamond water is actually quite compressible.

I think diamond has the highest known bulk modulus of normal solids. I'd imagine more esoteric states of matter like degenerate matter would have a (much) higher bulk modulus, but it would still not be incompressible.

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Did you mean the volume of electrons and quarks to be exactly zero? –  Godparticle Apr 24 at 18:12
    
I'm pretty sure that black holes are the only incompressible things. At least, I cannot think of anyway to compress one. –  RBarryYoung Apr 24 at 21:57
    
The bulk modulus is 1 over the compressibility (as defined in the answer by Kyle). –  Bernhard Apr 26 at 5:52
    
@Godparticle: yes, the volume of electrons and quarks is exactly zero. Well, it is assumed so in QFT. In String Theory they would have a non-zero minimum size. –  John Rennie Apr 26 at 5:54
    
@Bernhard: oops, thanks :-) –  John Rennie Apr 26 at 5:54

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