Final answer:
Molecules continue to move until they reach dynamic equilibrium where there is no net movement of molecules from one area to another.
Step-by-step explanation:
Molecules move constantly in a random manner, at a rate that depends on their mass, their environment, and the amount of thermal energy they possess. This movement accounts for the diffusion of molecules through whatever medium in which they are localized. A substance will tend to move into any space available to it until it is evenly distributed throughout it. After a substance has diffused completely through a space, removing its concentration gradient, molecules will still move around in the space, but there will be no net movement of the number of molecules from one area to another. This lack of a concentration gradient in which there is no net movement of a substance is known as dynamic equilibrium. While diffusion will go forward in the presence of a concentration gradient of a substance, several factors affect the rate of diffusion.