Reverse Osmosis

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Please be patient as you read this, it takes some "preparing the stage" in order to present reverse osmosis. Thanks.

This graphic shows osmosis after it has been completed. If an osmotic setup is left alone, it will eventually stop and the water column on one side (the right, in this example) will not rise any higher.

The reason it will not rise higher is because of the pressure exerted by the water column is affecting the water movement at the semipermeable membrane.

Before going on, the ChemTeam wishes to make one point very clear. Water will continue to move across the semipermeable membrane. What happens under the conditions above is that movement is in equal amounts IN BOTH DIRECTIONS. There is NO net movement of water in either direction.

When osmosis takes place, there is a net movement of water from the low solute concentration side to the high solute concentration side.

OK, the pressure of the water column is what stops the osmosis. However, it does not have to be a column of water. Anything which supplies pressure in the proper amount will cause the osmosis to stop. To explain this, let's first go back to the initial conditions. Here is the graphic you've seen before:

Now, it might be helpful to assign some concentrations that will result in the right side going up, so we'll let side A be 1.0-M and side B will be 2.0-M. Both are sucrose. Side A has a low solute concentration (or high solvent concentration) compared to side B. So osmosis would happen and the right-side solution level would go up and the left-side would go down.

Is there a way to stop the osmosis other than letting it stop naturally. The answer is yes! Here's the graphic:

Using the piston, we exert pressure right on the surface of the solution. At just the exact amount, the two solution sides will remain equal in volume and no osmosis will occur. Pretty neat, huh?

Suppose we exert an amount of pressure GREATER than that needed to stop osmosis. What happens? The answer is that we will get reverse osmosis. We will be able to push water in the OPPOSITE direction that it wants to go.

Is this a good thing?

Go to Reverse Osmosis Examples

Go to Osmosis

Go to the Osmosis Equation

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