Diffusion is the technical name for the process in which particles or molecules of different substances get mixed up due to molecular movement. The particles are in continuous random motion, they have high kinetic energy and collide with each other numerous times. Here the particles  move from an area of higher concentration to an area of lower concentration eventually resulting in intermixing of the particles in a short time.
Examples- i) mixing of water and ink without any stirring.

 ii) diffusion of perfume in air, the smell of which reaches you  through                                       air in a short time. 
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