In this demonstration, the blue dye, methylene blue, has a higher molecular weight than the compound potassium permanganate. Smaller, lighter molecules diffuse faster than larger, heavier ones, and the potassium permanganate solution (on the right) has diffused further from the well than the methylene blue (on the left).
Osmosis is the diffusion of water across a membrane.
In this demonstration, we put dialysis bags of either water or a sucrose solution into beakers that contain either water or the sucrose solution:
1. water in water
2. water in sucrose
3. sucrose in water
4. sucrose in sucrose
If we consider distilled water to be 100% water, and the sucrose solution to be 90% water and 10% sucrose, then we can see that there is a concentration difference between water and sucrose in beakers 2 and 3, but that there is no concentration difference in beakers 1 and 4. We should see no net movement of water or sucrose in beakers 1 and 4, but we should see a net movement of water iout of the dialysis bag in beaker 2, and into the bag in beaker 3. Osmosis will occur in beakers 2 and 3.
We can also refer to the relative concentrations of water and sucrose in the solutions as hypotonic, isotonic, and hypertonic. In a hypotonic solution, the concentration of solute (sucrose, in this demonstration) is lower than in the solution you are comparing it to. So in beaker 2, the water in the dialysis bag is hypotonic to the sucrose solution in the beaker. In a hypertonic solution, the concentration of solute (sucrose, in this case) is higher than in the solution you are comparing it to. So in beaker 2, the sucrose solution in the beaker is hypertonic to the water in the dialysis bag. In beaker 4, the solution inside the bag has the same solute concentration as the solution in the beaker. The solutions are isotonic. Now, for practice, describe the relationships of the solutions in beakers 1 and 3. Check your answer.