Diffusion is the process in which molecules move from a higher concentration to a lower concentration. This process happens at random.
Diffusion can happen in a gas or liquid but cannot take place in a solid object. A familiar example of diffusion is what happens when a smell travels throughout a room. The particles that make up the smell are in high concentration in one area until they begin to disperse, or diffuse throughout the entire room.
While studying diffusion, Scottish chemist Thomas Graham developed the law of effusion. Effusion is the rate of escape of a substance through a small hole or pore.