# Are liquids compressible?

If we take a cyclindrical container, fill any liquid upto certain level and then put a piston such that it touches the surface of water. If we start compressing the liquid, will the liquid compress? Assuming the walls to be perfectly rigid...this questoin arised from the topic ,"are liquids compressible"?

## Popular Answers

Iris Andrea Pantle·a) Ma < 0.2 can be approximated as incompressible (means: it is rather unlikely - though not impossible - that compressibility effects appear and are strong),

b) Ma > 0.7 should/must be considered compressible (also liquids!!!),

c) in between it depends on the type of flow and the attached geometry whether compressibility effects are too strong to be neglected.

Iris Andrea Pantle·@Archak: I should add, that I always look from the perspective of moving fluids or movements in fluids. Considering non-moving fluids (i.e. macroscopically non-moving where local movements due to thermal effects or initial shock-type pulses or other forcing mechanisms might appear), one is recommended to dig into the basics of thermodynamics interdependencies between pressure, density, temperature etc. Still then, compressibility of liquids is clearly proved though it might be small. The pure existence of the speed of sound in continua comprising fluids and solids (in addition to and not to be mixed up with the particle velocity, might it be of fluctuating or transporting type) can be considered a proof.