You can do the following demonstrations in order to show the difference between osmosis and diffusion. You will need: a potato, a knife to cut it, some salt and some sugar, a tablespoon, three cups, some plastic wrap, a napkin and some tape.
Put a tablespoon of sugar into the center of a unfolded napkin and then fold up the napkin around the sugar and tape it shut so that no sugar and spill out. Put the napkin bundle into one of the cups and fill the cup with water. Cover with plastic wrap and let sit for two hours.
Cut the potato along it's width so that you can get two circular slices about 1/2 inch thick. Grasping the potato slices by their edges, they should be difficult to bend. Take one of the cups and add salt to the cup until the salt can no longer dissolve in the water. Take one slice and put it into the cup of salt-water. Take the other slice and add put it in the remaining up and fill it with plain water. Cover both cups with plastic wrap and let them sit for two hours.
After Two Hours
Remove the plastic wrap from the cup with the sugar bundle and then the sugar bundle and taste the water. What does it taste like?
It should taste sweet because the sugar should have found its way out of the napkin and into the water. The water molecules were able to travel through the tiny holes in the napkin and start filling the napkin with water. When in its solid form, sugar was too large to get through the tiny holes in the napkin, but as the water got inside the napkin, the sugar began to dissolve. When dissolved, the sugar molecules cold individually get through the holes in the napkin. Thus the sugar moved from inside the napkin, where the concentration of sugar was high, into the water outside the napkin, where the concentration was low. This is diffusion.
Diffusion-The movement of molecules from an area of high concentration to an area of low concentration.
Remove the plastic wrap from the remaining two cups. Take each potato slice, one at a time and holding them from the edges as before, try to bend them again. How does it bend as compared to the first time?
The potato in the glass of plain water should have been even more difficult to bend than before. A potato gets it's firmness or rigidity from the water that it in it. Diffusion allowed the water to move into the potato in the glass of plain water. You might even notice that the slice got a little larger. It got stuck a little in the bottom of our plastic cup.
The potato slice in the glass of salt water should, on the other hand, have become easy to bend and rubbery because diffusion could not occur, and in fact, this potato slice even lost water! The cells in the potato are surrounded by a semipermeable membrane and this membrane allows some molecules to pass through but not others. It allows the water to pass through but not the salt. Salt, however, is attracted to water, which is why is dissolves in water in the first place. The attraction between the salt and the water molecules is enough to pull the water out of the potato and into the salt-water solution. This is osmosis.
Osmosis-The tendency of a solvent to travel across a semipermeable membrane into areas of higher solute concentration.