Diffusion Time & Distance:
Osmotic vs. Hydrostatic Pressure


This simulation illustrates a common classroom lab experiment: a sodium chloride solution is placed in an inverted funnel that subsequently is lowered into a beaker of distilled water. The base of the funnel is sealed with a semi-permeable membrane that separates the salt solution from the pure water. As osmosis proceeds, the net movement of water is from the beaker into the funnel, causing the height of the salt solution to rise in the funnel. Since osmosis is a slow, diffusive process, the fluid level rises over many hours so students are not able to witness the change. We can now “see” the increased concentration of water in the beaker compared to the funnel, and watch the net movement of water across the semi-permeable membrane. By “witnessing” the movements of molecules you can then understand the reason for the rise of the water column, as well as its eventual endpoint as a balance is struck between osmotic and hydrostatic pressures.