And eventually we will have an equilibrium that will exist and our concentration gradient will basically cease to exist. Now, according to Brownian motion, molecules A and molecules B will still move across our semipermeable membrane, but there will be no net change. So we take six molecules A and place them on the left side of a fluid filled container. We also take six molecules B and place them on the right end of our container. These two sides are separated by semipermeable membrane that allows both of these molecules to move with ease. Now, entropy dictates that after a while the most mathematically probable case will be a system in which we have these two molecules even out.