When a cylinder of colourless hydrogen gas inverted over a cylinder of brown bromine vapour, after some time, we can see that both the cylinders become yellowish brown. This means hydrogen has travelled to the lower cylinder and bromine vapour moved to the upper cylinder.
Gases have the tendency to intermix and to form a homogeneous mixture. This property is known as diffusion.
Diffusion is defined as the process of intermixing of two or more gases, irrespective of density relationship adn without the help of external agency.
Graham's Law of Diffusion: The rate of diffusion of a gas is inversely proportional to the square root of its density or molar mass.
Effusion: Effusion is a special case of diffusion wherein a gas escapes through a small aperture from the vessel in which it is contained.
The rate of escape is inversely proportional to the square root of its density or molar mass.