Why can air be compressed but not water?
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Water is like a bucket of marbles — all the molecules are in contact and sliding over each other. Since they’re in contact, there isn’t much of a way to make them get closer.
Air (or water vapor) is like marbles spread out on a plate (or in a box, but that’s slightly harder to visualize) — there’s space between them and they’re bouncing around. With air, the molecules are excited and bouncing off each other, and that’s what makes it have volume. But if you squeeze it into a smaller space, they get closer and bounce around more. That’s also why it heats up when you compress it.
There’s a lot of space between molecules in a gas, so you can just cram them closer together.
Liquids and solids are at maximum cram and don’t have any more space to lose, so they resist significant compression until the molecules themselves or the container fails structurally.
Water can be compressed, it just needs very high pressures, and then it doesn’t compress very much. Go a mile deep in the ocean, and even at that high pressure it compresses by some fraction of a percent. But if you take liquid water and compress it too much, it turns to ice.
*Everything* can be compressed. Gases just have the property that, for a given amount of material and pressure, the volume and pressure are inversely related. To be more ELI5: doubling the pressure halves the volume, and vice versa.
Solids and liquids instead have a “bulk modulus” which defines how pressure causes compression. The number for water is 2.2 GPa and for diamond it’s 443 GPa. If you want to compress a substance by, say, 1%, you need to apply a pressure of 1% of its bulk modulus, 22 MPa (3200 psi) in the case of water.
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