An ideal gas still exerts pressure, even though in theory it has no interactions between particles. Roughly speaking if you put a bunch of bouncing particles i

An ideal gas still exerts pressure, even though in theory it has no interactions between particles. Roughly speaking if you put a bunch of bouncing particles in a box then even if they don't hit each other they're going to hit the walls of the box more often if you decrease the volume. This means pressure is proportional to density so inversely proportional to volume i.e. pV is constant.