So let's look at an illustration of diffusion. Suppose I have a cylindrical tube and I have two claps. Suppose I take this clap, I soak it into ammonia NH three and plug it up. Suppose I take this cloth, I soak it up in hydrochloric acid and then plug it up as well. So it's plugged up on both sides and I have air molecules in the middle. So orange guys are air molecules, green guys armonia molecules and red guys are hydrochloric molecules.