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Diffusion is the process where particles spread out from an area of higher concentration to an area of lower concentration, driven by the random motion of particles. This phenomenon occurs primarily in liquids and gases because, in these states of matter, the particles have enough energy to move freely and collide with one another.

In gases, particles are far apart and move rapidly, allowing for quick and efficient diffusion as they mix with one another. In liquids, while the particles are still closer together than in gases, they can slide past one another, also facilitating diffusion, albeit at a slower rate than in gases.

In contrast, solids have tightly packed particles that vibrate in fixed positions; diffusion is significantly limited in solids because the particles do not have the freedom to move around. This is why diffusion does not typically occur in solids and is primarily observed in liquids and gases.