Diffusion
Diffusion, in physical science, the flow of energy or matter from a higher concentration to a lower concentration, resulting in a homogeneous distribution. If one end of a rod is heated or electrically charged, the heat or electricity will diffuse from the hot or charged portion to the cool or uncharged portion. If the bar is made of metal, this diffusion will be rapid for heat and almost instantaneous for electricity; if the bar is made of asbestos, the diffusion will be slow for heat and extremely slow for electricity (see Insulation ). Diffusion of matter occurs most rapidly in gases, more slowly in liquids, and most slowly in solids. The spreading of odoriferous molecules (a smell) throughout a room is a common example of gaseous diffusion. A solid may dissolve and diffuse through a liquid, as when a lump of sugar is placed in a cup of water. This process is much slower than the diffusion of a gas; if the water is not stirred, it may take weeks for the solution to become homogeneou