What cause diffusion in gas?

Answer 1

I'm sure you're aware that the vast majority of matter (aside from dark matter, which is unknown) is made up of very small particles called molecules. These molecules all have a bit of kinetic energy, making them move around (the measure of this kinetic energy is actually temperature).

In a situation in which diffusion occurs, for example, if 2 different gasses are introduced into a common environment by either joining the containers or releasing both containers into a third area, the gasses will move about the room in a more or less random manner (I'm sure you can visualize what would happen if someone threw a handfull of marbles across the room in an orbital platform of some kind...the behavior of gas will be much the same, but thrown in every direction simultaneously, at varying speeds...plus quantum effects, which are beyond me). As time goes on, the molecules, once clustered in near the exit of the bottle, will eventually become randomly spread out - diffused. The cause of this is the tendency for molecules to move about...vibration in solids or liquids, actual movement in gasses and dissolved substances. The cause of this movement requires a fairly deep understanding of particle physics.

Answer 2

Diffusion in gas is caused by random motion of gas molecules. Molecules move from areas of high concentration to low concentration in order to achieve equilibrium. This process occurs due to the kinetic energy of the molecules.