What does Fick's first law describe in mass transport?

• A. J = D (dC/dx); describes convective flux.
• B. J = −k (dC/dt); describes reaction rate.
• C. J is independent of gradient.
• D. J = −D (dC/dx); describes diffusive flux due to concentration gradient.

Answer: (D)

Fick's first law describes diffusion flux: how much substance moves per unit area per unit time due to a concentration difference. In one dimension, the diffusive flux is J = −D dC/dx. The negative sign shows that molecules move from high to low concentration, following the concentration gradient. The diffusion coefficient D encodes how easily the species diffuses in that medium and depends on temperature, the medium’s properties, and the size of the diffusing molecule. This law applies to steady diffusion, where the flux at a point is governed by the local gradient; if concentration changes with time, you’d use Fick’s second law for the time evolution. The other statements mischaracterize diffusion: omitting the negative sign, tying the relation to a time derivative or reaction rate, or claiming flux is independent of the gradient.