Answer: Particles are moving from regions of high concentration to low concentration.
Explanation:
Diffusion is driven by differences in concentration. When chemical substances such as perfume are let loose in a room, their particles mix with the particles of air. The particles of smelly gas are free to move quickly in all directions. They eventually spread through the whole room from an area of high concentration to an area of low concentration. This continues until the concentration of the particles of smelly gas is the same throughout the room. Remember that the particles still move, even when the smell is evenly spread.