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Osmosis is a special type of diffusion that applies to water and other solvents.
If you take a litre of pure water, and compare it to a litre of sea water, which is very salty, there is less water present in sea water litre because some of that litre is occupied by salt. The saltwater has a lower water potential.
If there is a partially permeable membrane, like a cell membrane, separating two different samples of water, there will be a net movement of water from the place with higher water potential to the place with lower water potential.
This is the reason plant roots take in water: the rain water the enters the soil has a higher water potential than inside of the roots, so water moves into the roots by osmosis.
We can demonstrate this in a lab setting using potatoes.
First you’ll need a cork borer to get your potato strips. Push it through the potato, then use the narrower piece to push the strip out.
For this experiment we need 18 strips. Once you’ve got as much as you need, tidy them up with a knife to remove the skins.
All strips need to be the same length to begin with. He we’re cutting them all down to 6cm.
Once they’re ready, get 6 beakers and put 3 tubes in each. This will give you three repeats for each solution.
Next thing to do is make your solutions. Put a sheet of paper on the balance, hit “tare” to get starting point of zero, and add the salt until it reads “1g”.
Once that’s in the beaker, you need 100 mL of water. Pour into the measuring cylinder keeping your eyes on level with “100mL” until it reaches the line. And then add that to your salt.
Here we’ve just made a salt solution of “1 g per 100mL”. In this experiment we need six solutions: 0g per 100ml, which is just pure water, and 1 to 5 g/ 100mL in 1 g increments.
Once the solutions are ready, pour them in to immerse the potato strips.
Start the timer, and leave them for 20 minutes.
After 20 minutes your have to measure all of the strips. Measure them as accurately as possible. To the nearest mm is good for this experiment.
As you’re working with the potato strips you probably notice that the once immersed in more concentrated solutions are more soft, which the ones immersed in lower concentrations are much more turgid.
Make a quick note of each value you measure as you’re going.
To process the data you need a table of results with all concentrations listed, as well as the starting lengths of the strips. These all started at 6.0cm.
In the next column put in the values your measured, and then calculate the percentage change for each individual strip. Notice the negative sign on some values, this tells us whether the strips increased or decreased in length.
Next you can take an average of changes in length. These are the values that we need to put on a graph.
The graph should show the average change in length against the salt concentration. And the results from this experiment show a nice downward curve.
The lower salt concentrations result in an increase in length of the potato strip, meaning water has ENTERED the potato strips by osmosis. This means there is a higher water potential inside the solution compared with the potato. We have a hypotonic solution.
At higher salt concentrations, however, we have a decrease in potato strip length. This tells us the water potential is higher inside of the potatoes, so water as left the potato strips by osmosis. We can say that the solutions are hypertonic to the potatoes.
At the point the graph crosses zero, which is about 1.4 g/100mL salt, the solution is isotonic, meaning the water potential inside potato is equal to the solution, to at that point there is no net movement of water.