In which state of matter doesn't diffusion happen?
It happens in all of them
Diffusion is a physical process where molecules of a material move from an area of high concentration (where there are many molecules) to an area of low concentration (where there are fewer molecules). It happens in every state of matter, even in solid. In XVII century Robert Boyle presented the process of diffusion in solid with the example of zinc and a copper coin. Another example is bluring of the colors of the glass or ceramic.