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#1
Old 02-13-2013, 04:43 PM
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Why is water basically incompressible? Are there squishy liquids?

See subjects. Is it a matter of all liquids?
#2
Old 02-13-2013, 05:01 PM
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Considering that all elements can be compressed to a liquid and then to a solid the incompressability is not a function of the liquid state. Liquid CO2 can be compressed into dry ice, for example.

Water acts weird because it's molecules are electrically polar; that is they have a positive end and a negative end (where the 2 hydrogen atoms are). That's also why water expands when it freezes. Each positive end has to line up with a negative end in a certain way.

Water is pretty fascinating stuff, when you compare it to other substances.
#3
Old 02-13-2013, 05:35 PM
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Most liquids have very small compressibility since there's not much space between the molecules to move closer. But they are compressible otherwise longitudinal sound waves wound not move through them. Compressibilty is basically an increase in density with pressure.

Here's a table of compressibility of normal liquids - http://engineering.uiowa.edu/~cf...bles/1-42B.pdf. You can see which liquids are more compressible and which are less.

Cryogenic liquids on the other hand have a density that is not much different from the gas and have greater compressibility. Liquid hydrogen will be the closest match in your search for "squishy liquid".
#4
Old 02-13-2013, 06:50 PM
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They are compressible, but they are routinely considered incompressible for many applications.

It's like calculating the momentum of cars traveling 60-80 mph. Any terms added to the calculation due to relativity are too small to have any major effect on the answer.
#5
Old 02-13-2013, 06:59 PM
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Quote:
Originally Posted by am77494 View Post
But they are compressible otherwise longitudinal sound waves wound not move through them.
Do longitudinal sound waves not also transit solids?

Last edited by eschereal; 02-13-2013 at 06:59 PM.
#6
Old 02-13-2013, 07:21 PM
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Liquids and solids are harder to compress than a gas because the molecules are essentially touching each other in liquids and solids. There is not much special about the compressibility of water, and it is easier to compress than many common materials.

For analysis of liquid flow, the liquid is often assumed to be incompressible because the tiny amount of shear force it takes to make the liquid flow is almost nothing compared to what it takes to compress to a comparable strain.
#7
Old 02-13-2013, 07:29 PM
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To the second question, "are there squishy liquids", the answer is by definition "no": A liquid is an incompressible fluid. There are, however, fluids which are neither liquids nor gases, and which would probably fit reasonably well with your mental image of "squishy liquid".
#8
Old 02-13-2013, 07:37 PM
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I guess if you want to test squishy liquids, start with some Jell-O?
#9
Old 02-13-2013, 07:48 PM
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I don't think Jell-O would be easy to compress. One of the most incompressible real substance is rubber, even compared to metals like titanium and steel, rubber is harder to compress, but it has a Jell-O-like lack of shear stiffness. To make a liquid more compressible, fill it with bubbles.
#10
Old 02-13-2013, 08:09 PM
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Quote:
Originally Posted by For You View Post
Do longitudinal sound waves not also transit solids?
yes they do - the longitudinal waves in solids are faster than the transverse waves. The point I was trying to make is that a medium needs to be compressible for compression (longitudinal) waves to propogate
#11
Old 02-13-2013, 08:14 PM
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Originally Posted by Chronos View Post
To the second question, "are there squishy liquids", the answer is by definition "no": A liquid is an incompressible fluid.
Well, yes and no. Liquids are often treated as incompressible fluids, and this approximation is close enough for the vast majority for most engineering and general science problems, but all liquids (and indeed, all substances short of degenerate matter) have some degree of compressibility within all states of matter. Some liquids will undergo may undergo partial thermodynamic state changes due to dynamic environments (e.g. vaporization due to cavitation) which has to be considered, and of course can expand and contract with thermal changes that may be the result of mechanical compression, although the volume change is very small. Solids are generally assumed to be one particular density, but can be permanently compressed to higher density. (I have held an inert steel "pit" that had been compressed by explosives to approximately three times the normal density of steel.)

Stranger

Last edited by Stranger On A Train; 02-13-2013 at 08:17 PM.
#12
Old 02-13-2013, 08:21 PM
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sorry

Last edited by am77494; 02-13-2013 at 08:22 PM.
#13
Old 02-13-2013, 08:32 PM
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Quote:
Originally Posted by Manlob View Post
To make a liquid more compressible, fill it with bubbles.
But then you're not compressing the liquid, you're compressing the bubbles.
#14
Old 02-13-2013, 08:33 PM
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Did that steel pit regain its original volume afterwards?
#15
Old 02-13-2013, 10:21 PM
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Quote:
Originally Posted by For You View Post
Do longitudinal sound waves not also transit solids?
Yes, because solids are compressible. Slightly.
#16
Old 02-13-2013, 10:33 PM
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Quote:
Originally Posted by Manlob View Post
One of the most incompressible real substance is rubber....


As opposed to a virtual substance, or an imaginary one?
#17
Old 02-13-2013, 10:43 PM
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Originally Posted by Stranger On A Train View Post
Some liquids will undergo may undergo partial thermodynamic state changes due to dynamic environments (e.g. vaporization due to cavitation)
I guess I always thought of cavitation as being primarily due to the dissolved gasses being beaten out of the water (thinking of propellers), but I could see that the back edge of a propeller might reduce the pressure in the water below its vapor pressure. I would assume that most of the water would recondense pretty quickly but some might bubble out?

Quote:
... and of course can expand and contract with thermal changes ...
Like alcohol or mercury in a thermometer.
#18
Old 02-13-2013, 11:01 PM
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Throw some water or other liquid into a black hole or neutron star and then tell me how incompressible it is!

The correct answer as given above is that the compressibility of most liquids is negligible and insignificant in nearly all applications.

Last edited by drewtwo99; 02-13-2013 at 11:03 PM.
#19
Old 02-13-2013, 11:16 PM
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I once worked in a place that did waterjet cutting. The water velocity at the nozzle was more than twice the speed of sound and the operating pressure to achieve that was 50000psi. The water compressed by around 12% in the process.

All substances can be compressed. However, for most contexts the compression of liquids can be considered to be negligible.
#20
Old 02-13-2013, 11:53 PM
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Since liquids compress so little, I wonder: does it take more energy to compress a gas than a liquid to the same pressure? Does that mean compressed gas tanks are more dangerous?
#21
Old 02-14-2013, 12:10 AM
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Originally Posted by AaronX View Post
Since liquids compress so little, I wonder: does it take more energy to compress a gas than a liquid to the same pressure? Does that mean compressed gas tanks are more dangerous?
See, here is the thing: red light! stomp on the brakes! What is happening there? You are trying to compress a liquid through a long skinny tube. Notice how little give it has? But what happens if the brake guy failed to properly bleed the air bubbles out of the lines? You end up compressing the gas bubbles instead of the oil.

Then, of course, there is propane. Propane gas can be compressed at normal temperatures to the point that it becomes liquid. This is kind of more-or-less true of any gas, but the vapor pressure for a lot of them is high enough that liquefying them is not very practical.

By the same token, if you put water into a vacuum chamber and started pumping out the air, you would reach a point where the water would actually start to boil at room temperature. I do not believe that there are any free liquids in space (outside an atmosphere) for this reason, they just turn into gasses.

Every substance has a state graph. One axis is temperature, the other is ambient pressure. It is commonly divided into three zones for solid, liquid and gas. As temperature goes up, liquids require greater pressure to remain liquid. And, of course, the three regions are adjacent, so there is a point, a combination of temperature and pressure, where a substance can freeze, evaporate and melt all at the same time.
#22
Old 02-14-2013, 02:54 AM
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Is that the "triple point"?
#23
Old 02-14-2013, 04:18 AM
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Quote:
Originally Posted by AaronX View Post
Since liquids compress so little, I wonder: does it take more energy to compress a gas than a liquid to the same pressure? Does that mean compressed gas tanks are more dangerous?
Yes. This is why pressure vessels (e.g. SCUBA air tanks, rated for ~2000 psi) are often subjected to hydrostatic testing (instead of compressed-gas testing): they fill it with water and then apply pressure. If the vessel ruptures during the pressure test, there's far less energy released than if compressed gas were used.
#24
Old 02-14-2013, 06:42 AM
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Last edited by am77494; 02-14-2013 at 06:43 AM.
#25
Old 02-14-2013, 06:45 AM
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Originally Posted by For You View Post
Propane gas can be compressed at normal temperatures to the point that it becomes liquid. This is kind of more-or-less true of any gas,
Bolding mine.

I think you mean "any gas below the critical temperature"; generally a gas above critical temperature cannot be liquified by pressure alone. Some define vapors as gas phase fluids below the ciritical temperature - so CO2 at room temperature is a vapor.
#26
Old 02-14-2013, 08:21 AM
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Originally Posted by Machine Elf View Post
Yes. This is why pressure vessels (e.g. SCUBA air tanks, rated for ~2000 psi) are often subjected to hydrostatic testing (instead of compressed-gas testing): they fill it with water and then apply pressure. If the vessel ruptures during the pressure test, there's far less energy released than if compressed gas were used.
That's what I was thinking. Compressing to the same volume change, however, is a different matter.
#27
Old 02-14-2013, 09:19 AM
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Originally Posted by For You View Post
I guess I always thought of cavitation as being primarily due to the dissolved gasses being beaten out of the water (thinking of propellers), but I could see that the back edge of a propeller might reduce the pressure in the water below its vapor pressure. I would assume that most of the water would recondense pretty quickly but some might bubble out?
Your second hypothesis is the correct one; some point on a propeller or impeller is moving so fast and in a direction contrary to the flowstream that it basically shears the fluid away from itself, leaving a vacuum and causing the fluid at the boundary to vaporize. This may also cause dissolved gases to come out of solution or some chemical kinetics in the fluid, but it can occur in a completely monolithic fluid with no dissolved air. The majority of the damage done isn't by the formation of the vacuum itself (most materials used for propellers and impellers can tolerate vacuum) or even the pressure differential per se, but rather by collapse of the void which often occurs with large high frequency shock content and is periodic in nature; for an impeller in a high speed centrifugal pump it may occur hundreds of times a second. This can be hugely destructive as it may cause both erosion of the nearby surface of the impeller and vibrational resonance that may couple to modes in the shaft or other nearby components (fatigue of springs, erosion of valve seats, aeroelastic flutter in flexible structures, et cetera) that can ultimately result in wear or catastrophic resonant failure of the system. It also contributes to significant losses in the system, which can dramatically reduce pump efficiency; in a centrifugal pump, cavitation may actually stall the flow and drive the effective output of the pump to zero, so impeller design and pump sizing for the application are crucial.

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#28
Old 02-14-2013, 10:13 AM
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As a practicing Chemical Engineer, I have often found folks to have several half-truths about cavitation. Here is a good paper to dispel these general myths.

Interesting Facts (and Myths) about Cavitation
#29
Old 02-14-2013, 10:19 AM
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Originally Posted by Fish Cheer View Post
Is that the "triple point"?
Correct. The Celsius scale used to be based on distance between freezing and boiling points of water-- but that depends on the atmospheric pressure so there had to be a subsidiary definition of "one atmosphere" of pressure. So now the Kelvin scale is based on distance from absolute zero to the *triple point* of water, both of which are non-arbitrary (and Celsius is simply an offset from Kelvin).
#30
Old 02-14-2013, 07:55 PM
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Quote:
Quoth Stranger on a Train:

Well, yes and no. Liquids are often treated as incompressible fluids, and this approximation is close enough for the vast majority for most engineering and general science problems, but all liquids (and indeed, all substances short of degenerate matter) have some degree of compressibility within all states of matter.
I agree with the meat of your point, but would differ on the semantics. I would say rather that ideal liquids are completely incompressible, but that all real liquids are (at least slightly) nonideal.

Oh, and degenerate matter (such as makes up a white dwarf or neutron star) isn't incompressible, either, and there is in fact a lot of work that goes into attempting to determine the equation of state of neutron star matter.
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