Diffusion

Particles in liquids and gases are constantly moving in different directions. When there are more particles in one region than in another, their random motion produces an overall spread from the more concentrated region to the less concentrated region. This net movement from higher concentration to lower concentration is called diffusion. Concentration describes how many particles are present in a given volume.

Diffusion and random particle motion

Kinetic particle theory describes matter as made of tiny particles with kinetic energy. These particles are always moving and frequently collide. They do not travel in a single stream from high to low concentration: individual particles move randomly in all directions.

Particles move both from the higher-concentration region to the lower-concentration region and in the reverse direction. However, because there are more particles in the higher-concentration region, more leave it over time. The result is a net movement towards the lower-concentration region.

Diffusion towards an even distribution

Diffusion continues until the particles are evenly distributed through the available space and their concentration is equal throughout. The particles do not stop moving: they continue to move randomly in both directions. At this stage there is no net movement in either direction.

Diffusion does not happen because particles ‘want’ to spread out. The overall spreading is a result of their continuous random motion.