The rate of diffusion is higher in gases, gaseous particles are in a constant state of motion, moving at random speeds and in numerous different directions. This is not only due to higher velocity of gas particles but also due to their smaller size, which enables them to pass in out of relatively smaller gaps and nooks. The particles move from an area of higher concentration to an area of lower concentration which eventually results in mixing of both the gases. Diffusion in gases is also known as effusion.
examples- i) mixing of perfume in the air.