This is why if you take a drop of food coloring and place it in a container, you can watch the diffusion process happen fairly quickly. A long time ago, a group I was in was tasked with measuring something just like this. We designed an experiment, and even modified glassware (sealing it) to minimize external factors. We were so confident that our design would yield the results that most closely mirrored the theoretical results. In the end, we were off by a very large factor, say 1000x or something, because we left the light from the spectrometer on, which increased the temperature on that point of our apparatus. Otherwise, our professor praised our design.