Question

give reason that gas undergo diffusion very fast

Answer 1

The force of attraction in the gas is quite low; therefore, the molecules have complete freedom to move around. Gasses easily take the shape of the vessels and tend to occupy free space. Hence, they undergo diffusion quite fast.

Answer 2

Gases undergo very fast diffusion as the intermolecular spaces between the particles of gases is very less..when the gases gain kinetic energy their molecules move very fast....resulting in the day diffusion. Not only this their density is also very low...which is also a cause for fast diffusion.
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