why liquids are considered “incompressible,” while gasses and solids can be compressed?
In: Physics
Liquids are compressible, although eventually with enough pressure they will just turn into a solid. Very rarely do you do something with a liquid where it’s compressibility is relevant to the calculation.
With something made from solid materials, it’s compressibility (and tensile properties) governs how it bends and squashes in each axis and its important to the structures stiffness. Its easy to put huge forces through small amounts of material, so how it compresses and stretches is very relevant. With liquids, the compressibility only matters when you directly compress them and not many things need to run at particularly high pressures.
Air pressure at sea level is about 1 bar.
At the bottom of the mariana trench the water pressure is about 1000 bar, water is about 5% denser.
Typical hydraulics on diggers run run at about 300-400 bar, compression of the oil is only a couple of percent.
In high pressure fuel systems on diesel engines which run about 2000-2400 bar, how much the fuel compresses is pretty important. This is by far the highest pressure everyday application I can think of.
Water jet cutters run at over 6000 bar. Water has a decent volume change at this point.
Liquids can be compressed, generally easier than solids. It’s just it’s a common assumption to make calculations with liquids easier because the error is pretty small. We generally often assume solids are incompressible when working with lower pressures.
Pure solids are less compressible than their corresponding liquids. But solids also often have pores or other open spaces (think foam) that can be squeezed out before you’re dealing with a pure material, which is probably why you think of solids as “compressible”. But something like a block of iron really isn’t.
Liquids, by contrast, don’t have open spaces because they’d just flow into them. And while we refer to liquids as “incompressible”, they are in fact compressible to a small extent: it’s just that that extent is small enough as to be practically zero for most engineering purposes. Even at the bottom of the ocean, seawater is compressed by only like 1%.
You can formalize this idea with something called the *bulk modulus*, which tells you how much pressure it takes to compress something. Most solids have higher bulk moduli than most liquids, but liquids’ bulk moduli are still pretty high and closer to solids than to gases.
Latest Answers