Why can air be compressed but liquids can’t?
In: Physics
You can compress them a little bit, but it’s generally much harder than with a gas.
One of the things that makes a liquid a liquid, and not a gas, is that the molecules in a liquid interact with each other strongly. In a gas, molecules mostly don’t interact unless a couple happen to collide and bounce off each other. But in a liquid each molecule is connected to its neighbors by intermolecular interactions. This is especially notable with water molecules hydrogen bonding to each other, but other things do it to.
These interactions are why if you put liquid into a container, it hangs out at the bottom instead of expanding to fill the whole container like a gas would. The molecules are basically stuck together
In order to have these interactions in the first place and be classed as a liquid, molecules have to already be pretty close to each other so you can’t easily squeeze them closer together. And the molecular forces try to keep them a certain distance apart, causing them to resist squeezing further. In contrast, there’s a lot of extra space with gasses and you can squeeze them until the molecules are much closer together (at which point they often start interacting and become a liquid).
So liquids are not very compressible, while gasses are very compressible.
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