When does a substance stop moving down its concentration gradient?

Question

When does a substance stop moving down its concentration gradient?

Answer 1

I assume the context of this question is diffusion.  The substance will move down its concentration gradient from a region of high concentration to a region of low concentration.  Over time, the concentration falls in the region of high concentration, and rises in the region of low concentration.  The rate of diffusion depends on the concentration gradient.  As the falling and rising concentrations converge, the rate of diffusion drops, and converges to zero when the two concentrations are equal.  There is no more gradient and diffusion stops at the time that the concentrations are equal.