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WHAT IS DIFFUSION
Diffusion is one of the fundamental processes by which material moves. It is thus important in biology and medicine, chemistry and geology, engineering and physics, and in just about every aspect of our lives. Diffusion is a consequence of the constant thermal motion of atoms, molecules, and particles, and results in material moving from areas of high to low concentration. Thus the end result of diffusion would be a constant concentration, throughout space, of each of the components in the environment.

Of course there are also processes that generate inhomogeneity, even while diffusion is smoothing things out, and the world we live in is the sum of both. The speed of mixing by diffusion depends on three main parameters:

    1. temperature
    2. size (mass) of the diffusing particles
    3. viscosity of the environment

        The temperature of a system is a measure of the average kinetic energy, the energy due to movement, of the particles in the system (the distinction between particles, molecules, and atoms will not be important in our discussion). In most systems the energy is just equal to a constant times the temperature. Since a higher kinetic energy means a higher velocity, it's clear why the speed of diffusion increases with temperature: everything is moving faster (in the formula below, E is the kinetic energy, k Boltzmann's constant, T the temperature, m the mass, and v is the velocity).

A heavy particle has a lower velocity for a given kinetic energy, or temperature. A large particle interacts more with its environment, which slows it down. Thus, heavy, large particles diffuse more slowly than light, small ones.
        
The environment (the material the diffusing material is immersed in) is very important. Diffusion is most rapid in a gas (because molecules can travel a considerable distance before they hit another molecule, and even then they just bounce off), slower in a liquid (there is a lot of movement, but all molecules remain weakly tied to each other as they move), and very slow or sometimes zero in a solid (because the forces between molecules and atoms are so generally so large that there are only infrequent exchanges of position).


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