What is diffusion?

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valentin68's profile pic

valentin68 | College Teacher | (Level 3) Associate Educator

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Diffusion is a process in which molecules or atoms of two (or more different substances) mix together because of their kinetic energy (thermal agitation). Diffusion can take place between the molecules different gases, different liquids and even between the molecules of different solids (on very short distances). In the last case, the two solids that are initially in contact weld together after a long time. The  rate of diffusion is proportional to the difference of the absolute concentrations of the two substances.

`(d(rho(x,t)))/dt = D*(d^2(rho(x,t)))/dx^2`

where `rho(x,t)` is the density of the given substance.

The factor of proportionality is named the diffusion constant `D` and it is proportional to the temperature of the medium in which the diffusion takes place.

`D = mu*k_B*T`

where `mu` is the molecules mobility of the given substance and `k_B` is the Boltzmann constant.

ayl0124's profile pic

ayl0124 | Student, Grade 12 | (Level 1) Valedictorian

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Diffusion is the movement of molecules across a semi-permeable membrane. There is no energy involved because it is a type of passive transport. The molecules move from "high to low" or from high concentration to lower concentration. 

An example of diffusion in biology is when small molecules such as O2 diffuse across the lipid bilayer. 

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givingiswinning | Student, Grade 10 | (Level 1) Valedictorian

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Moving from a higher concentration to one of a lower concentration.