all molecules have energy that causes thermal motion. one result of thermal motion is diffusion: the tendency of substances to spread out evenly in the available space. although the motion of each individual molecule is random, there can be directional motion of an entire population of molecules. consider a chamber containing two different types of dye molecules, purple and orange. the chamber is divided into two compartments (a and b) by a membrane that is permeable to both types of dye. initial