Which statement about diffusion flux J is accurate?

• A. J equals the diffusion coefficient times the negative of the concentration gradient.
• B. J = -D (dC/dx). J is diffusion flux.
• C. J equals the diffusion coefficient.
• D. J depends on temperature only.

Answer: (B)

Diffusion flux is described by Fick’s first law: the amount crossing a unit area per unit time is proportional to the negative gradient of concentration. In one dimension, that is J = -D dC/dx, where D is the diffusion coefficient (positive). The negative sign shows diffusion moves from higher to lower concentration, so the flux points opposite the concentration gradient. This makes the statement J = -D (dC/dx) the precise expression for diffusion flux.

A couple of related ideas help solidify the concept: the diffusion coefficient D sets how quickly species diffuse in a given medium and can depend on temperature, but the flux itself is not determined by temperature alone—it also depends on how steep the concentration gradient is. Also, expressing J as just D would omit how much diffusive transport occurs, since the gradient is essential to quantify the actual flux. In more general terms, J = -D ∇C in multiple dimensions.