Gases assume the shape and volume of a container.
Gases are the most compressible of all the states of matter.
Gases will mix evenly and completely when confined to the same container. Gases have lower densities than liquids or solids. It was the first state studied in detail. It is the easiest state to understand - for the purposes of general chemistry, there is a simple mathematical equation for the general description of a gas which works well in the real world, over a certain range of temperatures and pressures. The study of gases and its results formed the primary experimental evidence for ur belief that matter is made up of particles.