Osmosis demonstration. Apparatus used to demonstrate the movement of molecules through osmosis. Here, Visking tubing containing a strong sugar solution coloured with food dye has been attached to a clear capillary tube (top) and submerged in a beaker of water. Osmosis is the net movement of solvent molecules across a semi-permeable membrane in a direction that equalises the solute concentrations on either side. As the sugar solution is more concentrated, water passes into the tubing. This causes an increase in the volume of liquid in the tubing, increasing the pressure and forcing the coloured solution up the capillary tube, as shown here.