Question: It is true that liquid nitrogen needs to be very cool, but why can't we compress it so much?

Craig Fisher answered on 5 Feb 2017:

Yes, nitrogen needs to be cooled to below 77 K (= -196 oC) to liquify. Liquids are much more difficult to compress than gases because the atoms are relatively close to one another. This is especially the case at low temperatures, where the atoms have little thermal energy and can fit more snuggly together. If you apply a large enough pressure, the liquid will become a solid, which then becomes even more difficult to compress because its atoms are packed together even more closely, typically in an ordered array.