Diffusion is the of particles from one region to another. It occurs because of two properties: the particles have between them and are able to . If we start off with two different particle types which are separated, and then the barrier which is separating them then will occur. The particles from both regions will and move in the spaces between other particles. The particles of each type will continue to mix until they are distributed. Diffusion can only happen with and because solid particles move around.