1. The problem statement, all variables and given/known data Show that concentration of a substance obeys the diffusion equation if the rate at which substance leaves a region is proportional to the concentration gradient. 2. Relevant equations Diffusion equation: [tex]\nabla[/tex]2[tex]\phi[/tex] = (1/a) (d(phi)/dt) where Phi is a function of r, t. 3. The attempt at a solution I'm having trouble even starting this question. I think I have to integrate concentration (C(r,t)) over a volume to get total amount of substance, but there might be some other constants I have to throw in as well... and I'm not sure what they would be... I have a vaguely similar solved example to do with temperature, but I'm having trouble relating it to concentration. I don't really know where to begin! I'm not looking for a full worked solution, but could someone just give me a push in the right direction? Thanks very much!