Question

Why can't liquid change volume but gas can

Answer 1

Because the molecules in liquids are more tightly packed than gases, they are restricted to their container. Gases, on the other hand can move a lot more freely due to the extra space.

Answer 2

Particles are far apart from each other because there are very weak forces of attraction between them. Because of this, gases don't have a definite shape orvolume and fill any container. Because there is lots of free space between particles,gases can easily be compressed.