Diffusion.

     The word diffusion is derived from the Latin word, diffundere, which means "to spread out". Diffusion from its definition describes the movement of solute particles through a medium, from a region of higher concentration to a region of lower concentration. In other words, the solute particles tend to move from a more crowded region to a less crowded region. Once the solute particles become evenly distributed throughout the medium, there will be no net diffusion in any particular direction. 

    Diffusion in gases; Diffusion is fastest in gases because gas particles have more kinetic energy than particles in liquids and solids. The rate of diffusion of gasesis affected by their densities. 
   Diffusion in liquids; Diffusion also takes place in liquids, though it is much slower than in gases. This is because liquid particle have less kinetic energy and are more restricted in their movement due to cohesive forces between the particles. 
   Diffusion in solids; Diffusion may be observed in solids but the process is extremely slow. If a gold bar and a silver bar are tied together firmly and left for years, some gold particles will have diffused into the silver bar, and vice versa. 
        Diffusion is fastest in gases (in seconds). Their particles are far apart and the cohesive forces between them are negligible. It is slowest in solids (in years), where the particles are held in fixed positions by strong cohesive forces.