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If a container like a balloon but with constant volume is filled, is it possible to pack air molecules so closely together that they don't have any empty space between them? If so, what would this material look like?

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So what you're asking is basically: if you press enough gas molecules into a constant volume, would it always remain a gas? – leftaroundabout Apr 30 '13 at 23:37
Air is a material thing, and it looks like, well, air. – dmckee Apr 30 '13 at 23:53

Short answer: No.

As you pump more and more air into the container, the pressure rises and rises. At some point, the molecules are so close to each other that instead of a gas, you get a liquid. If you continue even more, eventually you'll get a solid.

In this solid, atoms/molecules are arranged in a regular pattern with well defined distances between them (which depend on the pressure), but those distances are not zero so technically there's still "empty" space between them.

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Take this beyond into the ridiculous, and you'll end up squeezing out the space between the electrons, neutrons, and protons, and you'll end up with something akin to the material that neutron stars are made of. – Benjam Jul 3 '14 at 16:00

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