Final answer:
When diffusion stops, the solution has reached equilibrium, a state where the concentration of the substance is equal throughout the space, resulting in no net movement of molecules.
Step-by-step explanation:
When diffusion stops, the solution has reached equilibrium. This process yields a uniform dispersal of molecules throughout a space, known as dynamic equilibrium. At this point, there is no net movement of molecules from one area to another because the concentration of the substance is balanced throughout the space.
In biology, when considering cell membranes, equilibrium is achieved when molecules flow equally in both directions across the semipermeable membrane, with equal amounts entering and leaving a cell. This state is reached when the concentration gradient no longer exists, meaning that the concentrations of the substance on both sides of the membrane are equal, and there is no longer an area of higher or lower concentration.