Why is water incompressible?
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Water is compressible. You can look up the bulk modulus, which essentially tells you how much pressure you need to compress an object by a certain amount.
Water has a bulk modulus of 2.1 GPa. For comparison, air has 142kPa (about 10’000 times less). Rubber about 2 GPa. Steel and Aluminum have about 160 GPa, a lot more than water.
The reason we often hear water is incompressible is that for almost all applications, its compressibility does not matter. The pressure needed to force water through pipes, or even the pressures experiences in a hydraulic press, is much lower than yhe pressure needed to significantly compress water. So in almost all calculations, the compressibility of water can be completely ignored.
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