In diffusion, what does the diffusion coefficient DAB represent?

• A. The proportionality between a chemical's flux and its concentration gradient
• B. The ratio of convective to diffusive mass transfer
• C. The diffusion distance per unit time
• D. The saturation concentration of A in B

Answer: (A)

Diffusion coefficient D_AB measures how readily A moves through the B medium. It is the proportionality constant in Fick's first law, linking the diffusive flux to the concentration gradient: J_A = -D_AB ∂C_A/∂x. This means that for a given concentration gradient, a larger D_AB gives a larger flux of A, while a steeper gradient increases flux as well. D_AB encapsulates how easily molecules diffuse, depending on temperature, solvent viscosity, and molecular size or interactions with the solvent. It’s not about convective to diffusive transfer, not a diffusion distance per unit time (distance scales with sqrt(D t)), and not a saturation concentration. Its units are m^2/s.