Learning Objectives:

• Differentiate between steady-state diffusion, where concentrations do not change with time, and unsteady-state (transient) diffusion, where concentrations change with time.
• Understand and apply Fick's First Law to describe steady-state molecular diffusion in one-dimensional systems.
• Define the diffusion coefficient (or diffusivity) and understand its role in describing the rate of molecular diffusion.

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Molecular diffusion is the spontaneous movement of particles (molecules or ions) from an area of high concentration to an area of low concentration.

Fick's First Law:
Describes steady-state molecular diffusion. Mathematical Representation: \[J=−D \dfrac{dC}{dx}\] ​ J is the molar flux.
D is the diffusion coefficient (diffusivity).
$\dfrac{dC}{dx}$ is the concentration gradient.

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Solved Example: 9294-01

Which of the following statement is incorrect regarding diffusion?

A. The diffusing molecules move randomly.

B. Rate of diffusion of molecules is proportional to the density of medium through which they move.

C. Direction of diffusion of one substance is dependent on the movement of another substance.

D. Movement of molecules is due to their kinetic energy.

Correct Answer: C