Method:
Firstly, I will cut out the ten chips of potato that I plan to use. Using a cork borer, I will take them all from the same potato, and then weigh them to ensure that they all weigh exactly 1.68 grams, and cutting any excess off with a knife. Then, I will pour the sucrose solutions into ten test tubes, with two test tubes for each solution so that I can do repeats of my experiment. The concentrations of sucrose solution that I use will be 0.2, 0.4, 0.6, 0.8 and 1.0 mole dm-3, and I will use 10ml of solution per test-tube. Then, I will place the chips in the test tubes, cover the tops with cling film and leave for two hours and twenty minutes.
When I return, I will remove the chips from the solution using tweezers to ensure that I do not inadvertently squeeze out any of the water that the chips may have absorbed. I will place the chips onto a paper towel to remove any water droplets on the surface of the chips that could affect the mass of the chips, and then I will weigh them to find any changes in their mass, and record the results.
Prediction and Fair Testing:
For this experiment, I will be varying the concentrations of the sugar solutions that I place the potato chips in from 0.2 to 1.0 mole dm-3. In order to make sure that the experiment is a fair test, I will need to keep the following variables constant:
Temperature: I need to keep the potato chips and solutions at the same temperature. This is because molecules diffuse faster at higher temperatures, as they have more energy, and they diffuse more slowly in low temperatures because they have less energy, so the rate of osmosis increases or decreases with temperature. Also, selectively permeable membranes are delicate things, and they could be denatured in too high a temperature. Therefore, I will keep my experiment away from any windows, heaters and so forth so that the temperature of the chips and sucrose solution does not change and affect the rate of osmosis.
Time: Osmosis needs time to happen. In a longer period of time, there will be more time for osmosis to take place, and in a shorter period, there will be less time. Therefore, I am going to leave all the potato chips in their separate solutions for two and a half hours to ensure that the test is fair and all the chips have the same amount of time for osmosis to happen in.
Type of potato: I will cut all the chips out of the same potato. This is because every potato will have slightly different cells from other potatoes. The cells from different potatoes will have slightly different concentrations of water in their cytoplasm, and even slight differences in the concentration of water in the cytoplasm will affect the concentration gradient between the cytoplasm and the sugar solution, which will affect the amount of osmosis that needs to take place before the two solutions are the same concentration, and this would change very slightly between each different potato. In order to ensure that the tests I use are fair, I will continue to take all of the potato chips I use out of the same potato.
Concentration of sugar solution: The factor that I will change for this experiment is the concentrations of the sugar solution that I will place the potato chips in. I predict that the chips in the lowest sugar concentration will increase the most in their mass, and that the chips in the highest sugar concentration will decrease the most. This is because when a potato chip is placed in pure water it is surrounded by a higher concentration of water molecules compared to inside the cytoplasm of each cell. The water will diffuse into the cell by osmosis. This means the cytoplasm pushes out against the cell wall as it expands slightly. The pressure inside the cell increases. The cell walls in a plant are strong enough to withstand this, though, so all that happens is that the cell becomes firm or turgid. The overall effect over time is for each cell to increase slightly in mass and so the whole potato chip shows an increased mass. However, if you put a chip of potato into a strong sugar solution (where the water concentration is lower than the water concentration in the cytoplasm in the potato’s cells), then water will move out of the cells of the potato. The potato chip will therefore shrink, and become floppy, or flaccid. If a sugar solution has exactly the same water concentration as the potato piece, then it is likely that there will be no overall movement of water in or out of the potato. Therefore, the potato piece will stay the same size.
I predict that a graph drawn of the changes in mass of the potato chips will be a curve. This is because the cells in the potato will either lose water or gain water by osmosis but the amount of water lost or gained from the cell is will not be proportional to the concentration of the solution it is in. If the cells gain water because they are in a solution where the concentration of water outside the cells is higher then the concentration of water inside the cells, then the cells of the potato chip will gain water until the cytoplasm can expand no more because of the cell wall, making the cell turgid. This means no more water can move in by osmosis. The same applies to the cells losing water by osmosis and becoming flaccid.
I have also made some changes to the method that I used for my preliminary experiments, in order to keep the experiment as fair as possible, and hopefully to receive better results.
In the previous method that I used, I measured the length of the chips with a ruler before and after placing them in the solution. However, this did not take into account all of the particle movement and cell expansion in width, and it was very easy to measure the chips inaccurately. Therefore, for my new experiment, I intend to measure the change in mass of the potato chips. This is a more accurate way of measuring all the cell expansion or contraction, as it can take into account even very minute changes in the potato’s cells and therefore the potato itself, and there is much less room for human error, as the scales are very accurate. It also means that I can ensure that at the beginning of the experiment, all the potato chips are exactly the same size. Ensuring that they are the same size at the beginning of the experiment is very important because it means that they should all have the same surface area and capacity for absorbing or exuding water. They will all initially weigh 1.68 grams.
I will use a size 4 cork borer to cut out the potato chips. This is because this method worked well in my preliminaries; using the same cork borer to cut out all the chips ensures that all the chips have the same width, and this method works well with my measuring of mass to ensure that all the potato chips have the same surface area as well, so that the experiment is kept a fair test.
In my preliminaries, I left the potato chips immersed in the solutions for thirty minutes. However, there was only a minimal change in the length of the chips after this time. Therefore, I will leave the chips immersed for a longer period- 2 hours and 20 minutes, which should allow osmosis to take place on a larger scale, helping me to have more conclusive results.
In order to spot any anomalous results, I will repeat the experiment. This will also allow me to find an average change in grams for each sucrose concentration.
In the last experiment, I left the test tubes containing the solutions and chips uncovered. However, I plan to cover them with cling-film for this experiment. This is in order to stop unwanted evaporation. If any of the water in the solutions evaporated, it would change the concentrations of the solutions and therefore affect my experiment. Wrapping cling-film around the tops of the test-tubes should be an effective way to stop this from happening, and keep my experiment fair.
In my preliminary experiment, the concentrations that I immersed the potato chips in were distilled water, ‘weak’ sucrose solution and ‘strong’ sucrose solution. These definitions, excepting the pure water, are very vague. In order to be able to draw a better conclusion, I will use five different sucrose concentrations. The concentrations I use will be 0.2, 0.4, 0.6, 0.8 and 1.0 mole dm-3. Not only are these concentrations more clearly defined, but they also are a much wider spread, which will aid me in finding patterns in the results. This should help me to gain more accurate results, draw an accurate graph of my findings, and also reach a clear conclusion.
When I did my preliminary experiments, I removed the chips from the solution with my bare hands. However, for this experiment, I will remove the chips from the solution using tweezers to ensure that I do not inadvertently squeeze out any of the water that the chips may have absorbed. Also, before measuring their mass, I will place the chips onto a paper towel to remove any water droplets on the outside of each piece that could affect the mass of the chips.
Results:
Analysis and conclusion:
explain what the line graph means. When it goes up, levels off, goes down. For a conclusion with your experiment you should first describe the results you have obtained, usually shown as a graph, identifying any patterns you found and then you should try to explain in detail what was happening to give you the pattern
As the concentration of the sugar solution increases, the mass of the potato chip decreases, which can be seen from my results tables and my graph. The points on my graph form a curve that where the height of the line decreases as the concentration of the mol dm-3 increases. Although on the whole my results are devoid of anomalies, my last results in both sets are very different. Because I have only got two sets of results, I cannot tell which is the correct result. I need to take another set in order to find out which result is the anomalous one.
As the concentration increases, the graph begins to level out slightly. This is because the potato chips can only lose a certain amount of water, and as the concentration of sucrose solution increases, the amount of water the cells lose gets closer and closer to the maximum amount possible for the cells to lose, so the line of the graph levels out.
However, on the whole my results support my prediction very strongly. The potato chips in the highest concentration of sucrose (and therefore the lowest concentration of water) lost the most weight, and the chips in the lowest concentration of sucrose (and the highest concentration of water) gained the most weight. This is because, in osmosis, water moves across a partially permeable membrane from a weak solution (high concentration of water molecules) to a stronger solution (lower concentration of water molecules). When the potato chips are in the lowest sucrose solution of 0.2 mole dm-3, the concentration of water molecules in that solution is higher than the concentration of water molecules in the cytoplasm of the potato’s cells. In osmosis, water always moves from a high concentration of water molecules to a lower concentration, and so the water moves from the solution into the cytoplasm inside the potato cells. This increases the pressure inside the potato cells, and the cytoplasm pushes out against the cell wall, making it become firm and turgid.
However, the cells which I placed in a higher concentration of sucrose solution – the chips in 0.4, 0.6, 0.8, and 1.0 mole dm-3, were surrounded by solution with a lower concentration of water molecules than in the cytoplasm of the potato cells. This meant that once again, the water moved from a high concentration of water molecules to a low concentration, except that this time, the high concentration was inside the cytoplasm and the lower concentration was the sucrose solution, meaning that the water moves out of the cytoplasm instead of into it. This meant that the cells were not as full of cytoplasm as usual because all the water in it was moving out of the cell, and they became flaccid. The cells of the potato became floppy and flaccid, and smaller. Because this effect was happening to all its cells, the potato chip also became floppy and decreased in size.
From my results, and using my graph, I can estimate approximately the concentration of water in the sucrose solution needed to balance the concentration of water in the potato cells- the isotonic point. If a sugar solution has exactly the same water concentration as the potato piece, then there will be no overall movement of water in or out of the potato and therefore the potato piece will stay the same size. My graph shows that at a concentration of 0.29 mol dm-3, there is no change in the mass of the potato chip. At this concentration, the concentration of water molecules inside the cells of the potato chip and outside balance out and no osmosis takes place.
Evaluation:
The points on my graph are not a wide scatter, but very narrow and close to the line of best fit. Although on the whole there do not seem to be any anomalies, my last set 2 result does not fit well with the rest of the graph. This could have been caused by changes of temperature. I needed to keep the potato chips and solutions at the same temperature. This is because molecules diffuse faster at higher temperatures, as they have more energy, and they diffuse more slowly in low temperatures because they have less energy, so the rate of osmosis increases or decreases with temperature.
The potato chips I used probably had some small differences in mass, which could also affect my results. A solution to this could be to calculate the percentage change in mass of the chips. This would mean that a more accurate change in mass for each chip could be calculated, which would make the results more accurate. The equation for this would be:
Mass at end – Mass at beginning x 100%.
Mass at beginning
I could not calculate the percentage change in mass for this experiment as I did not record the minute differences between the mass of the potato chips at the start of the experiment, and so if I wanted to calculate the percentage change in mass I would have to record exactly the slight differences in mass between all the potato chips at the beginning of this experiment.
I did keep my experiment away from any windows and heaters, but the room the experiment was in probably did warm up or cool slightly in the day. A possible solution to this problem could be to keep the experiment in a water bath or other controlled environment. It would be difficult to find the temperature of the sucrose solution without removing the cling film, however, which would allow some water to evaporate and also affect the experiment.
It would also be helpful to have collected more sets of results, as this would allow me to find more accurate averages, and also would help me to spot any anomalous results more easily; with only two results, it can be difficult to tell which result of the two is anomalous. In future, I will try to collect three or four sets of results which should be a great help.
It would also be good to have a control experiment where a potato chip is placed in distilled water. This would be useful to get a wider spread of results. I could also try using 0.1, 0.3 0.5, 0.7 and 0.9 mole dm-3, which likewise would give me a wider spread of results to help me draw improved conclusions and graphs.
Overall, I think that my results were accurate and useful, and my method was fair, although I could have improved my control of temperature by placing the test tubes in a controlled environment such as a water bath, and I should have taken more results.
