If we add a drop of food colour into two beakers, out of which one is filled with water and the other with milk. So, In which case does the food colour will diffuse faster? 

Diffusion is the mixing of molecules due to their random motion. The more space to do the random motion, the more will be diffusion. The water particles have more space in between them than milk particles. Thus, the food colour will diffuse at a higher rate in the case of water.