The sugar molecules are too large. In the tube, the sugar molecules interfere with the water molecules and prevent some water from moving through the membrane. As a result, more water moves into the tube than out of it. As water moves into the tube, the sugar solution rises, thereby increasing the pressure in the tube. The solution continues to rise until the pressure that it exerts equals that of the pure water.
Chemists use a process called reverse osmosis to purify water and perform other functions.
In normal osmosis, water flows from fresh water into seawater when they are separated by an appropriate semipermeable membrane. But, by applying pressure to the seawater, chemists can reverse the movement of the water and produce fresh water from seawater. John B. Butt, D.Eng., Former Walter P. Murphy Professor of Chemical Engineering, Northwestern University. How to cite this article.