Diffusion is the net movement of molecules from a region of high concentration to a region of low concentration. It occurs due to the random motion of molecules. Even when diffusion stops, the molecules are still in motion, but their net movement is zero.
This can be understood by considering a beaker of water. If some salt is added to the water, the salt molecules will start to diffuse throughout the water. The salt molecules will move in all directions, but the net movement will be from the region of high concentration (near the bottom of the beaker) to the region of low concentration (near the top of the beaker). Eventually, the salt will be evenly distributed throughout the water, and the diffusion process will stop. However, the molecules in the water are still moving, even though the net movement is zero.
