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.
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.
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.
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.
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.
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).
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.