Question

Why gases do not have definite volume?

Answer 1

Gases do not have a definite volume because their atoms have large intermolecular spaces. They have high kinetic energy which makes it impossible to remain at a place and random motions are provoked. They are highly compressible, so you can compress them to whatever extent you want. The gases tend to escape in open and closed systems. So you can't have a specific volume of them.