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he kinetic molecular theory for gases says that the molecules are in continuous straight line motion and that their kinetic energy is greater than the attractive forces between them. So they move freely and independent of each other, and they are much further apart than a solid or liquid. It also says that there is no energy loss when a molecule collides with another molecule or the side of a container. This explains everything in your list.
Expansion: The molecules are moving in continuous straight lines. So they will continue in one direction until colliding with something, meaning they will fill whatever container they are in.
Fluidity: Fluidity is the ability to flow, or move from one place to another. Since gases are in constant motion, and the molecules are independent of each other, the gas is able to flow.
Low Density: The molecules are much further apart than with a solid or liquid. So per unit of volume there is very little amount gas, as compared to a liquid or solid. Density is mass over volume.
Compressionability: Is this even a word? Gases can be compressed because there is so much space between molecules.
Diffusion: Diffusion is movement from an area of high concentration to an area of low concentration. Gases are in constant motion, so it follows that if you have an area of high concentration, the gas will move to the area of low concentration. You can see this work when you open a container of compressed gas. If diffusion didn't exist, then the compressed gas would just stay in the container