Question

Why do liquids and gases differ in density and the ability to be compressed?

Answer 1

Because gases are mostly empty space.

Explanation:

Now it is fairly easy to compress a gas; you can do so in a very simply piston; in fact we do so when we pump up a bicycle tyre. This variable density supports the notion that gases are mostly empty space. On the other hand, condensed phases, solids and liquids, display very little variation in volume under various pressures and temperatures. Both liquids and solids are incompressible to a first approximation.