What is described by a rate of diffusion?

The rate of diffusion, dn/dt, is the change in the number of diffusing molecules inside the cell over time. Since the net movement of diffusing molecules depends on the concentration gradient, the rate of diffusion is directly proportional to the concentration gradient (dC/dx) across the membrane.

my teacher would say no s

yes, because even though a theory has a  lot of evidence to support it, it is not always true