What is diffusion and how does it occur?

Answer 1

Diffusion is the movement and spreading of substance throughout its location. For example if you spray perfume on one side of the room the scent will spread throughout the entire room.

Mass transfer is similar because its the movement of particles from areas of high concentration to areas of lower concentration. Going back to our perfume example, there was a high concentration of perfume particles where you sprayed the perfume and a low concentration on the other side of the room, the molecules moved from the area with a high concentration to the area with a lower concentration.

Answer 2

Diffusion is a transport process where one type of material moves through another driven by thermal motion. Intrinsic in diffusive motion is the character that the particle moving does so in a basically random fashion, changing both the direction and speed of movement based on collisions with other particles in the medium.

To get a little more technical, there some related terms like, simple diffusion, passive diffusion, active diffusion, active transport, self diffusion and facilitated diffusion. These are related and some clarification will be added below, but generic diffusion as addressed here is the movement of atoms or molecules driven entirely by the normal thermal kinetic energy that all particles have as a consequence of temperature. Thus, diffusion, simple diffusion and passive diffusion all mean the same thing. The term self diffusion refers to the same process of particle diffusion as a consequence of temperature but in the case that the movement take place among other particles of the same type.

With diffusion, when two materials come into contact, the atoms at the contact point begin to intermingle just as a consequence of the normal thermal motion that all atoms have. In the process, there is mixing at the interface. The two sets of atoms continue to move and eventually the rather random moving and bumping gets atoms from their starting point further and further into the other material. Since this is basically random, some atoms move away from the interface and some towards.

Eventually, with diffusion, one will get uniform mixing. When the concentration gradient is absent, atoms continue to diffuse, but there is no net transport since net transport is always from areas of high concentration to low concentration.

(Caveat: Uniform mixing refers to the disappearance of all concentration gradients. It does not mean that all materials will uniformly mix with all others. Oil and water do not mix completely. At the interface between immiscible liquids, there is a force that tends to push particles back into their "normal" place. This can only be explained within the context of forces between atoms and molecules, so it is beyond the current question.)

Diffusion is then nothing more than the natural motion of an atom or molecule through some medium as a consequence of its translational kinetic energy.

Diffusion occurs in gasses liquids and solids, being naturally slow in solids and fast in gasses. Because diffusion is a consequence of the intrinsic kinetic energy of atoms and molecules, the higher the temperature, the fast the motion and the more rapid the diffusion.

The other terms, active diffusion, active transport, and facilitated diffusion imply the presences of an additional force that moves molecules other than just the normal kinetic energy associated with temperature. These terms have meaning that tend to reflect the use in more specialized areas such as Biology. The discussion here has been directed to generic diffusion as a form of transport of atoms and molecules.

Final Caveat: One sometimes hears about the diffusion of heat. Since heat is not a particle, the above discussion does not apply. It is however true that heat diffusion obeys the same equations that describe particle diffusion. Heat diffusion is, of course, based on particle movement, but in that case the movement does not involve transport substantially, just local interactions transferring kinetic energy (hence temperature) to adjacent atoms and molecules and that energy continuing to propagate to more distance atoms and molecules through random collisions.