Osmosis
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Osmosis

A phenomenon that is related somewhat to the change in freezing point, the change in boiling point, and the change in vapor pressure of solutions when compared to pure solvents, is the process of osmosis. I'm sure you've heard of it. I'd like you to take a look at it now (using the pictures on this pages) and later when you are in the lab. I also recommend that you record your observations in exercise 18 in your workbook.

This osmosis apparatus contains a pure water separated from a solution by a thin membrane. So we have three things: a solution, a membrane, and pure solvent. Water is the pure solvent in this case. Osmosis apparatus.
By looking at the change in liquid levels, you can see that something has moved from the water, through the membrane, into the solution. If something came from pure water it had to be water molecules because there was nothing else there. Water level raised in tube by osmosis. (Start of demo)
pointer marks initial position

2 minutes
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Water level raised in tube by osmosis. (2 minutes elapsed)
top of tape marks initial position

 

So the water passed through the membrane from the water side to the solution side. Actually, water passes through the membrane in both directions but it moves faster into the solution than out of it. Water can go in either direction, but the solute particles within the solution cannot pass through the membrane. What seems to happen is that the presence of the solute particles restricts the flow of water molecules from the solution into the pure solvent. But they are not able to restrict the motion of the pure solvent into the solution. Consequently, the water passes from the pure solvent to the solution in greater amounts than water molecules from the solution pass into the pure solvent. Diagram of water molecules moving through membrane.

 

So, the water passes through the membrane in both directions, but it passes through at a higher rate from the pure solvent into the solution, than it passes from the solution into the pure solvent. That process is called osmosis. Another aspect of osmosis is something called osmotic pressure. As solvent molecules pass through the membrane into the solution, they build up pressure. If the side with the solution were closed off and had a pressure gauge mounted on it, you would be able to read the osmotic pressure generated by the flow of water into the solution.

Here is another way of describing osmotic pressure. The flow of water into the solution can be stopped by applying pressure to the side where the solution is. The amount of pressure needed to stop the flow is the osmotic pressure. Actually the flow of molecules is not stopped, but the flow is equal in both directions and cancels out.

It is not necessary for one of the liquids to be pure water in order for osmosis to occur. All that is necessary is for the concentrations on the two sides of the membrane to be different.

Isotonic Solutions

In the diagram shown here the dotted line represents a semipermeable membrane through which water molecules (but not solute particles) can pass. The small dots represent water molecules and the larger red dots represent solute particles. Note that the solute concentration is the same on both sides of the membrane. The solutions are said to be isotonic compared to one another. Isotonic solution diagram.
Because the solute concentrations are the same on both sides of the membrane, water molecules move through the membrane equally well in both directions. There is no net flow of water in either direction. Isotonic solution diagram showing migration of water molecules.

 

Hypertonic Solutions

In this diagram the solution on the left side of the membrane has a higher solute concentration than the solution on the right side of the membrane. The solution on the left is said to be hypertonic compared to the one on the right. Diagram of hypertonic solution.
The higher solute concentration on the left reduces the flow of water molecules from left to right, causing a net flow of water from the right to the left. If the right side represented a cell placed in a hypertonic solution, water would leave the cell causing it to dehydrate and collapse. Diagram of hypertonic solution showing migration of water molecules.

 

Hypotonic Solutions

In this diagram the solution on the left side of the membrane has a lower solute concentration than the solution on the right side of the membrane. The solution on the left is said to be hypotonic compared to the one on the right. Diagram of hypotonic solution.
The lower solute concentration on the left allows for increased flow of water molecules from left to right, causing a net flow of water from the left to the right. If the right side represented a cell placed in a hypotonic solution, water would enter the cell causing it to swell and perhaps burst. Diagram of hypotonic solution showing migration of water molecules.

 

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