Osmotic pressure is particularly easy to understand on the basis of escaping tendencies. It is illustrated by the diagram opposite, which shows a single glass tube with one end covered by a semipermeable membrane, and immersed in a beaker of water.

With pure water on both sides of the membrane, the escaping tendency of molecules through the membrane from either side is the same.

Now if molecules of some solute that cannot pass through the membrane are added to the tube, but not to the beaker, the escaping tendency of water molecules from the tube is decreased.

Although the rate of flow of water molecules through the membrane into the tube is unimpeded, the reverse flow from tube to beaker is hindered, since not every molecule that approaches the membrane from the inside surface will be a water molecule.

If only 90% of the molecules in the tube are H2O, then the flow of water out to the beaker will be only 90% as great. More water will flow in than out, and the water level will rise in the tube.