Dear Student, Diffusion is the process in which particles of solute moves from higher concentration to lower concentration. Particles of gas are loosely packed so diffusion of particles in gaseous medium is high. In liquids, the particles are loosely bound but not as much like gas, so diffusion in liquid is slower than gas. Particles of solid are compactly arranged they only vibrate along their position.So, diffusion in solids is very slow. Gas >Liquids>Solids. Regards