There is another way, however it is not as accurate, You could carry out the proposed experiment draw up the graph, and using it, you could draw in the estimated equilibrium point as I have done.
Once the potato tuber in the test tube has reached the same ratio of sugar: water as the solution surrounding it the process will stop as osmosis will only take place where there is a separation of low and high concentrate, there fore there will be no change in weight.
How ever if the situation is reversed and there is a lower concentration of sugar in the potato than there is sugar in the solution (as shown in diagram 3.) The process is reversed as well. This is called plasmolysis. Instead of the cells in the potato swelling they 'shrink' causing the whole potato to decrease in mass.
Equipment;
- 6 clean test tubes.
- 6 potato tubers weighing 2 grams each
- 20ml of 0M, 0.2M, 0.4M, 0.6M, 0.8M, 1M of sucrose solution.
- Stop watch.
- Knife.
- Filter paper.
- Cork
- Test tube stand
- Scales
Procedure;
- Measure out 20ml of 0M, 0.2M, 0.4M, 0.6M, 0.8M, 1M of sucrose solution. *
- Pour these in to 6 different clean test tubes.
- Cut out 6 potato tubers carefully weighing them to exactly 2 grams each.
- Place one in each test tube also start the stop watch and put a cork in the test tube to stop evaporation.
- Leave it standing for about 24 hours, no more than this other wise there is a risk of the potatoes becoming too soft to handle.
- Come back to the experiment; pour the solution through a filter to make sure you don't loose the potato you are examining.
- Reweigh the potato and record the weight
- Redo the experiment above.
- Compared the two differences in weights.
* How to work out how much of each liquid you have to put in:
You want to make 20ml of 0.0M
0ml (sugar) + 20ml (distil water)
= 20ml of 0.0M
You want to make 20ml of 0.2M
4ml (sugar) + 16ml (distil water)
= 20ml of 0.2M
You want to make 20ml of 0.4M
8ml (sugar) + 12ml (distil water)
= 20ml of 0.4M
You want to make 20ml of 0.6M
12ml (sugar) + 8ml (distil water)
= 20ml of 0.6M
You want to make 20ml of 0.8M
16ml (sugar) + 4ml (distil water)
= 20ml of 0.8M
You want to make 20ml of 1M
20ml (sugar) + 0ml (distil water)
= 20ml of 1M
Safety;
Although this is a fairly safe experiment we have to be aware of the potential dangers. When using the razor blades we will have to be very cautious, as it is easy to slip and cut your self. We will make sure that we know where the blade is at all times and return it to the place where we got it from.
Incase of test tube breakages, we will notify the teacher straight away, and get it cleared up before any thing else happens. We will avoid handling the broken glass when we can. We will also be wear safety glasses to avoid anything coming in contact with our eyes.
We will make sure that the surface and the space around it is dry and tidy before we carry out the experiment as the water may affect the results and it will be easier to locate things when we need them. We will also check the floor if it is wet, to avoid any one slipping up whilst rushing about to get equipment.
Variables;
To make sure that this will be a fair test we will be taking precautions. To make sure that we will not loose any water through evaporation or let any unwanted substances into the solution with the potato we will cover the opening of the test tube with a cork.
From a similar experiment I have done previously I have learnt that just taking measurements of the potato is not accurate as the potato expands both lengthways and out wards during this process. Hence why this time we will be recording the mass, as opposed to the length. We will record the weight before and after the experiment to compare the increase or loss of mass. This will be more accurate.
Results;
From these results we can see that the more water in the solution, in ratio to the sugar, the more weight is put on by the potato. This is caused by osmosis. Osmosis is the movement of water from a low concentration to a high concentration through a semi-permeable membrane. When there is more sugar, in ratio to the water, weight is lost. This is because the water that is already present in the potato is passed through to the solution. This is called plasmolysis, the reverse action to osmosis. These two processes are shown in diagram 2.
We can also tell that the point in where no transaction is made is between 0.2M and 0.4M of sucrose solution. I have marked out the rough estimate of this point on the graph.
If a solution is separated from the pure solvent by a membrane that is permeable to the solvent but not the solute, the solution will tend to become more dilute by absorbing solvent through the membrane. This process can be stopped by increasing the pressure on the solution by a specific amount, called the osmotic pressure- but we have not studied this variable yet. (Information obtained from Britannica CD 1999)
Conclusion;
My prediction is correct. We did not obtain any anomalous results. I think the results that we obtained clearly show that there is a marked difference in the process; which takes place within the potato when put in different concentrations of sucrose solution. The consistency in the results suggests that the apparatus selected for this experiment was appropriate.
I have already suggested a way to find out the approximate intake of water by the potato in my prediction.