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. So basically, we have the same molecules A on this side as on that side and the same number of B molecules on this side as on that side. We conclude that molecules tend to naturally move from a high concentration to a low concentration. So given situation A, we say that there exists a chemical concentration gradient and both molecules travel down their perspective, chemical concentrate their respective chemical concentration gradient.