Why is water incompressible?


Why is water incompressible?

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Where’s it going to go? Most fluids are not compressible because they’re appear as close as they’re going to get with each other. There’s a tiny bit of space, and most liquids will compress a little, including water, but it’s not enough that we measure the difference and consider it in any meaningful way.

No. Water as with all materials can be compressed. But it is far less compressible than gasses, such that with normal pressures it isn’t useful to consider how much it changes in volume.

You can know that water can be compressed because sound waves exist within water. Since sound is a pressure wave you know by extension water must be able to be compressed by some amount.

It isn’t literally, more just for all practical purposes. The pressure required to compress a liquid is much greater than an equal volume of a gas because at the molecular level, the particles are already very close together. In order to force them closer, a vast and impractical amount of pressure would be required. It’s not impossible physically, just mostly impractical and unnecessary for most applications.

Water is not incompressible, no material on Earth is incompressible. Water has similar compressibility to chalk. In most circumstances, the amount it compresses is negligible, but in high pressure hydraulic systems compressibility of the working fluid is quite an important factor.

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.

Liquid water forms a lot of hydrogen bonds with each other. Due to the shape of the water molecules, you can’t really squeeze the molecules any closer together without snapping these hydrogen bonds, which takes a lot of pressure to do so.

However, with enough pressure, you can indeed snap those bonds and force water into a denser configuration. However, since water is already pretty dense to begin with, even this ‘denser configuration’ isn’t that much denser than what you started with. Eventually it becomes so dense that it solidifies, although not into the normal type of ice that we know (which is actually less dense than liquid water)

If you look at a phase diagram of water, you’ll see that compressing it at room temperature (~300K) will eventually turn it into Ice VI, then Ice VII, then Ice X, then Ice XI