Osmosis is the diffusion of a solvent through a partially permeable membrane until there is an equal concentration on either side of the membrane. The partially permeable membrane contains pores that are very small. Because these pores are only very small, only certain molecules can pass through it. Usually though the membrane will allow all or none of the molecules through. When only one type of molecule is let through though, the membrane is said to be partially permeable. This is because it allows a selective flow. A classic example of this is Visking tubing. If a concentrated solution of...
hypertonic. When exhibiting equal osmotic pressure a solution can be said to be isosmotic or isotonic. In our example of the Visking tubing, if we count the sugar molecules as the main component of the solution we can say that the water was the hypotonic solution as it contained the least sugar molecules and that the sugar solution was hypertonic because there were a greater number of sugar molecules. After osmosis occurs and the water molecules have been exchanged, the solution would have been isotonic as there would have been an equal amount of concentration on both sides.