The next step was for each group to perform the experiment, each group in a different way. My group (Me (Steffan), Rhys, Hazel, Holly, Daniel. J) decided to use a size 3 borer, and record the results of 2 different concentrations 0.0 M and 1.0 M, we decided to use potato strips that were 4 cm in length, and used plastic dishes to hold the potato strip, and the concentration of salt solution. We also decided to use 20 cc of Solution in each dish.
Here are my results (with percentage change worked out) :
Now here are some class results:
The results obtained are in the table, as shown above. We can see by looking at the table, and the two best results (1 and 3) that a smaller size borer is better, because in results 1 and 3 a size 2 borer is used and the percentage changes are close in both sets of results for a molarity of 0.0 M.
We also learnt that instead of plastic dishes, glass test tubes should be used, because the potato strips were not totally submerged in the concentration liquid, so instead glass test tubes were decided upon as a better choice, because they totally submerged the potato strips. Another reason why test tubes would be better is because a cork can be stuck in the top of the test tube, reducing evaporation, while the plastic dishes were not covered at all, which increased the risk of more inaccurate results. We also learnt that instead of measuring the length of the potato strip each time, mass would be recorded each time and the length of potato strip kept constant, as it is very hard to measure each potato strip to the exact mm accurately.
So overall the method that should be employed:
Small Size borer used (preferably size 2),
Glass test tubes with airtight cork to be used,
Mass of each potato strip to be recorded,
Length of each Potato strip kept constant each time.
Actual Method
For the actual method I shall be keeping some factors constant to obtain the best results possible
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The factors I shall be keeping constant are :
Size 2 borer,
Length of potato strips 3 cm,
20 cc of solution to be used,
Same sized glass test tubes to be used,
2 test tubes for each concentration labelled A and B (1 Strip of potato for each test tube, then average percentage change worked out for each concentration).
The factors I shall be varying are:
Concentration (salt solution) in each test tube(to obtain a range of results).
When conducting this experiment there are certain safety aspects that should be taken into consideration:
Wear safety goggles in case some solution gets into your eyes, and could cause some serious harm,
Do not carry the cutting knife/scalpel around the classroom; also take care when cutting the potato, in case harm is caused.
When finished with knife/scalpel return it to a safe place, out of harms way, do not leave it lying around on the desks etc.
In all I will be testing 6 concentrations :
0.0 M,
0.2 M,
0.4 M,
0.6 M,
0.8 M,
1.0 M.
There will be 2 test tubes for each concentration labelled A and B, so in all there will be 12 test tubes. 12 strips of potato shall be cut with a size 2 borer, and at a length of 3 cm, and a strip of potato placed in each test tube. But before being placed in the test tube each potato strip must be weighed on an electronic scale and recorded in a table. After the experiment is complete each piece of potato shall be dried and then re – weighed, and the new weight recorded. All in all I will have 4 weights for each concentration (2 weights per test tube, 2 test tubes for each concentration labelled A and B), one weight for test tube A before experiment, and one weight for test tube B before experiment = 2 weights. Ten we will have one weight for test tube A after experiment, and one weight for test tube B after experiment = 2 weights.
I will be using 20 cc of solution, here is a table showing how I will be setting up all the different concentrations:
List of apparatus: Salt solution, Water, Measuring Cylinder, Size 2 borer, Knife/scalpel, Potato, Cutting tile, 12 glass test tubes, Test tube holders (Rack), Sticky labels, Electronic Weighing scales, Ruler (For measuring length of potato).
Results:
Now I shall work out the percentage change in mass of the potato strips (to 2 decimal places) using the following method (for each test tube).
Final mass – Starting mass x100 = Average % change in mass
Start Mass
There will be 2 percentages for each concentration, I will then work out the average change in mass of the two test tubes.
Here are all the workings out:
0.0 (A) 0.86 – 0.73 x100 = 17.81%
0.73 Average = 19.32%
0.0 (B) 0.87 – 0.72 x100 = 20.83%
0.72
0.2 (A) 0.53 – 0.68 x100 = -22.06%
0.68 Average = -18.78%
0.2 (B) 0.60 – 0.71 x100 = -15.49%
0.71
0.4 (A) 0.41 – 0.63 x100 = -34.92%
0.63 Average = -32.25%
0.4 (B) 0.50 – 0.70 x100 = -29.58%
0.70
0.6 (A) 0.52 – 0.72 x100 = -27.78%
0.72 Average = -30.33%
0.6 (B) 0.49 – 0.73 x100 = -32.88%
0.73
0.8 (A) 0.54 – 0.71 x100 = -23.94%
0.71 Average = -25.61%
0.8 (B) 0.48 – 0.66 x100 = -27.77%
0.66
1.0 (A) 0.56 – 0.73 x100 = -23.29%
0.73 Average = -22.13%
1.0 (B) 0.49 – 0.62 x100 = -20.97%
0.62
Evaluation –
We can see by looking at the graph and the table of results that as the as the concentration of salt solution in the water increases, the mass of the potato strips in the water decrease. This proves my hypothesis is correct, and the process of osmosis has taken place. On the whole my results were pretty good. We can see this by comparing two results, which are the average % change in mass at O.2 M and the average % change in mass at 0.4 M, which is double the concentration. At 0.2 M the average % change was –18.78% and at 0.4 M it was –32.25%. By comparing these two results we can see that the difference in the two concentrations is double the other (0.2 M – 0.4 M) and the average % change of at O.4 M is fairly close to double the Average % change at 0.2 M which proves that the quality of our results were pretty good.
There was no change in mass at 0.10 M, this represents that the concentration was equal inside the cell and outside the cell and therefore there was a steady net flow in both directions and there was no % change. At 0.0 M the percentage change in mass is 19.32%, this is because osmosis makes plant cells swell up if they’re surrounded by weak solution and they become turgid. In pure water (0.0M) the potato strips swell because water enters their cells by osmosis.
From 0.1 M onwards water leaves the cells due to osmosis, because the concentration outside the cell is lower so water begins to leave the cells, because osmosis is the movement of water from a region of high water concentration to a region of low water concentration. By looking at the graph we can see that as the molarity increases more water leaves the cells due to osmosis, because the concentration outside the cell is lower than it is inside the cell.
Some results are a bit suspect and we can see this by looking at the graph, because the % change in mass it starts to rise during the last three molarities, Which are 0.6 M, 0.8 M, and 1.0M. when the line on the graph should be starting to straighten out it starts to rise. This is because there is quite a difference in the average % change which are at 0.6 M, -30.33%, at 0.8M, -25.61%, and finally at 1.0M, -22.13%. There is a vast difference in these results when they should be fairly close together. I think the reason for this is that when drying the potato strips some have been rolled in the tissue a little harder than others and water has been lost/squeezed from the potato. Another reason could be that when re weighing there was water left on the scales from another group, which would have given my group an inaccurate reading. Another reason could be when using the borer to cut the potato it could have been pushed through at an angle which would have given 1 strip of potato a larger surface area than the other, and therefore could absorb more water and it would also weigh more than the other potato strips. These are just a few factors that could have caused a rise at the end of my graph.
Improvements could have been made to the method, one would have been taking a wider range of molarities e.g. ) 0.0 M, 0.1 M, 0.2 M, 0.3 M, 0.4 M, 0.5 M, 0.6 M, 0.7 M, etc. which would have given me a wider range of results to compare with more accuracy.
Keeping certain factors constant was not too hard. The one constant factor I could not control was the temperature of the room were the test tubes were kept, as it was warmer in the day than the night which could have had a bearing on the results, but as they were all kept in the same place, in the same room they were all kept under the same conditions.
The measuring for concentration (cc), and Mass of potato strips, were done very accurately using a measuring cylinder for the concentration and electronic scales for the mass of the potato. The measuring was done to the most accurate degree possible, the only measuring were it was maybe slightly inaccurate was the cutting of the potato strips as there was no equipment to help us measure them accurately, so we relied on the accuracy of the naked eye, and a ruler.
Conclusion –
Overall the experiment was completed successfully, but if we were to re do the experiment we would ask for more time. As the time scale was very small it didn’t allow the group to cut sufficient amount of potato strips. Also, if I was to repeat the experiment I would make sure that all the potatoes used in the experiment were the same age, and species/brand to get a greater degree of accuracy in the experiment for better results. I could have also used a more accurate scale, which again would have provided a greater degree of accuracy for the optimum results. When pushing through the potato with the borer I could have been pushing through a hole that had already been used, which could give an inaccurate when weighing. And as we relied upon the naked eye when cutting the potato strips to length a degree of accuracy has been lost. If we were to repeat this experiment again I would try to ensure that I had the best modern technology available to get the most accurate set of results possible.
