Understanding Diffusion and Demonstrating it Correctly – A Critical Look at a Transport Process often Incorrectly Taught in Schools

Diffusion is a transport process frequently shown experimentally in beginning science classes. However, quite a few standard school “diffusion experiments” such as letting food colorant drip into water and spraying perfume (or similar) into a classroom provide a false picture of diffusion by demonstrating convection, transport by flow, instead. The article explains why these two standard experiments clearly show convection and should therefore be banned from school lessons as "diffusion experiments": Diffusion is explained to be an entropy-driven process and, based on this, the so-called Einstein-Smoluchowski equation is derived with the help of simple probability calculations. Using the Einstein-Smoluchowski equation, it is then shown how transport phenomena can be clearly recognized as diffusion or be unmasked as convection experiments. The reader is provided with alternative genuine diffusion experiments for beginning classes that verifiably demonstrate diffusion, namely the spread of ammonia and hydrogen chloride in a sealed glass tube and the spread of methylene blue in water-based gels. Additionally, it is shown how these experiments can be used to quantify diffusion constants with the aid of the Einstein-Smoluchowski equation.