There are lots of fun experiments and demonstrations for showing the movement of particles from a region of their higher concentration to a region of their lower concentration down a concentration gradient (diffusion). Here are some of the methods I use.
One demo that is often used is dropping purple potassium permanganate crystals into a basin or beaker of water and observing the slow dissolution and diffusion over time.
Cup of tea
Provide students with a tea bag (fruit tea bags are best because the colour change is more vivid) and a beaker of hot water (or water of different temperatures). Look at the factors affecting the rate of diffusion by telling the students that you are thirsty and want to speed up the time it takes to make your morning cuppa – how can they do this? (e.g. heat the water, put more tea into the tea-bags or teapot, reduce the volume of water).
Prepare agar plates and then ask the students to carefully cut a 1 cm wide moat around the circumference of the agar using a scalpel. Fill the moat with food colouring. The circle of agar represents a cell and the food colouring, the extracellular fluid. Over the course of the lesson the food colouring will slowly diffuse through the agar into the ‘cell’.
If you position a camera phone above the agar plate using a clamp stand and film using the time-lapse function you can capture and speed up the whole process to then show the students in summary at the close of the session. See video below:
The classroom will smell like a changing room but a very simple method of demonstrating diffusion is to spray deodorant in one corner and then ask students to raise their hands when they smell it. This creates a ‘Mexican-wave’ effect as the particles diffuse through the air.
The most effective way of demonstrating diffusion through a semi-permeable membrane is to fill dialysis tubing with starch and place it in iodine solution. The iodine will diffuse through the dialysis tubing (turning the starch blue-black) but the starch particles (being too big) will not diffuse in the opposite direction.
If anyone has any other fun demos of diffusion I would love to hear them!