An Explanation using Images

The Osmosis Equation

Reverse Osmosis

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The graphic just below shows a U-shaped tube. Notice that dividing line with the arrow pointing at it. What that line represents is REAL important. The two sides of the tube (A and B) have the same volume of solution at the start. For this example, side A has a 1.0-M sucrose solution, while side B is pure water.

The line with an arrow pointed at it represents a semipermeable membrane. This is the item that makes the osmosis happen. A semipermeable membrane allows solvent molecules to pass through it, BUT NOT SOLUTE!

That's important. Solvent can go either direction easily, but solute is stuck on the side it's on. It cannot move past the semipermeable membrane. This is because the holes in the membrane are too small for the molecules to get through.

By the way, sometimes molecules will be discussed and sometimes ions. IT DOES NOT MATTER what is in solution. Only how many items are dissolved, be they molecules or ions.

OK, back to the tube. Side A has a greater concentration of SOLUTE. That should be pretty easy, since poor side B doesn't have any solute at all. The key here is that the semipermeable membrane locks the solute in place. It is blocked from movement by the membrane.

So now lets look at the SOLVENT, which is water. (Keep in mind that the water can move across the membrane.) Which side has a greater concentration of water?

The answer is side B. In side A, some water molecules have been replaced by sugar molecules, so there are less water molecules in the same volume as side B.


The side that is LESS concentrated will have MORE water.

If the explanation is getting too long, then take a break for a moment. If you're at home, go get a drink or clean house for five minutes. Then help your dad revive your mom! If you're at school, take a big stretch and rub your eyes. Then, back to work.

OK, next point. There is a well-supported idea in science that things tend to move from areas of high concentration to low concentration. We'll leave off proving it, because, well frankly, you're not knowledgable enough yet. (The fact that the ChemTeam isn't either is beside the point!)

Well, in our example what can move? The solute can't because it's blocked by the semipermeable membrane. However, the solvent can move and it does. There is a net movement of solvent from side B across to side A.

The volume of side B will go down and the volume of side A will go up. That means the sucrose concentration will decrease from 1.0-M. Here is the graphic showing that:

For this next example, side A has a 1.0-M sucrose solution, while side B is 2.0-M sucrose. Remember the sides start out equal in volume.

The side with more solute is the side with LESS solvent. Water will move toward the side with the greater amount of solute. The result after some time is this:

The right-hand side will eventually stop moving up (and the left-hand side down). This is because the right-hand side's column is exerting pressure.

The column rise can also be made to stop by the application of mechanical pressure on the surface of the solution. This is discussed in the Reverse Osmosis file. The amount of osmotic pressure exerted can be measured and explained via an easy equation. This is discussed in the Osmosis Equation file.

The Osmosis Equation

Reverse Osmosis

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