Elementary Productions: Potassium Iodide and Lead Nitrate

A petri dish is filled with a small amount of water. Potassium Iodide is added to the petri dish in one end, and Lead Nitrate in the other end. As the two salts diffuse, Lead Iodide is precipitated as a "barrier" surrounding the place where the Lead Nitrate was placed. This is because of a substance with a high molecular weight diffuses more slowly than a substance with a lower molecular weight. This rule applies for any medium used, in this case water. Another good example of diffusion is why flatulence is able to smell up a room in seconds. Hydrogen sulfide (the substance responsible for the smell) has a pretty low molecular weight, and therefore diffuses faster. This experiment serves as a good demonstration on diffusion, because of the difference in molecular weights, the insolubility of Lead Iodide in water, and the color of the precipitate.