The types of potatoes we use are going to be kept the same, because different potatoes may absorb at different rates. For this experiment I am going to use the same brand throughout.
The time the potato chip is left in the solution must be kept the same in each experiment. This is because more or less solution may be absorbed depending on time. I am going to keep each potato chip in each solution for 10 minutes, in a beaker.
To make the mass readings more fair, I will take each chip of the solution, roll all the surfaces gently on a paper towel, to remove all excess solution, and I must not squeeze the chip.
I am also going to use the same balance to weigh my potato chips. This is because the measurements may vary slightly between scales.
I predict that the higher the concentration of salt solution the more water will move out of the potato chip. We know that osmosis the flow of one solvent, (water) of a solution through a membrane while the other constituents are blocked and unable to pass through the membrane. Experimentation is necessary to determine which membranes permit selective flow, or osmosis, because not all membranes act in this way. Many membranes allow all or none of the constituents of a solution to pass through; only a few allow a selective flow. As the potato is a plant cell, it contains a vacuole and a cytoplasm. The cell membrane is also partially permeable i.e. it lets some substances in but not all substances. This means that water can diffuse into cells by osmosis if the cells are surrounded by a weak solution. (Osmosis is the movement of water molecules from a high area of water potential to an area of low water potential through a partially permeable membrane.)
To ensure safety, beakers will not be placed near the edge of the table and one person will cut the potato chips very carefully and away from everyone else.
I will take readings of the concentration of the salt solution, which will have already been made. I will also have to measure each potato chip I use, and make sure that they are all of the same size. The mass of the potato chip will be weighed before and after the experiment to show whether water moved in or out of the chip, and also to what extent, (in grams). I will measure the time the potato chip is left in the solution with a stopwatch, and this will be measured in minutes, (10). The volume of solution the chip is put in can be kept the same because the chips are all of the same size, and seeing as they all must be totally covered, I will have each chip in 50ml of solution. I will cut 11 (multiplied by the number of repeats (3)) chips into equal lengths of 3cm. After I weigh them and record these results, I will place them into pre-labelled beakers of which are already containing 50ml of the relevant solution. I will place all the chips as quickly as I can and then saturate the stopwatch, for 10mins. I will then remove all the solutions from the test tubes as quickly as I can, (so that the chips do not carry on changing weight, and the test is kept as fair as possible), and I will then weigh them and record the results. These will then be put into tables and will be used in graphs. I will also measure the temperature in the room.
I am going to do the experiment three times, in each solution, and I will then take an average to have a fairer reading. One result may not be accurate because of human error, so to eliminate all possibilities of inaccuracy I will repeat it. I must also do at least 5 types of concentrations, to have sufficient results and to see if my prediction is correct, and I am going to do 7.
Here is the equipment I am going to use:
Test-tube rack
Test-tube filled
Beaker filled with solution
Stopwatch
Scalpel
Paper towel
Prongs holding chip
Results:
Analysis
There is a pattern on my graph, and this is, as the concentration of the salt solution increases, the percentage change in mass decreases. All of my results lie close to my best-fit curve. This shows that my results are fairly reliable. (Graph at back of Investigation).
The average distilled water gains in mass. This means that osmosis takes place and that the water moves from the beaker into the potato, matching my initial predictions. The point where the line crosses the x-axis is the isotonic point. This is where no osmosis is taking place, i.e. no water is moving in or out of the potato. The next point, 0.40ml looses approximately 0.02g. This shows that the water potential of the salt solution in the beaker is weaker than that of the potato chip. The next, 0.60 m, loses approximately 0.04g in mass. This shows that the salt solution’s water potential is weaker than 0.40 m and that osmosis took place. This is why the potato lost even more mass, and it shows that the water potential in the beaker is less than that of the potato chip. The potato is also trying to make the water potential of the fluid on both sides of the semi permeable membrane the same, (osmosis from the potato to the salt solution, from a higher water potential to a lower one). This pattern carries on through the graph, and even more mass is lost, as more water moves out of the potato. My results tally with my initial predictions. These were that osmosis would take place, and as the water potential decreases, concentration increases, the percentage change in mass will decrease.
Evaluation: The experiment was easy to do, but all the results I took had to be accurate. Also, I changed what the potato chips were going to be put in. I decided that it wasn’t sufficient to put them in the test tubes along with the solution, as even a little knock would ruin the experiment, as the solution was so near to the top. We then decided it would be better to keep them in a beaker.
I think I took enough results for the amount of concentrations that I used.
There were not any anomalous results because taking averages makes the experiment more accurate, but some results were not as close to the line as others. This may have been caused by human error, or one out of my three results could have been inaccurate, and changed the average drastically. Or perhaps the potato chip was not cut accurately, or that part of the potatoes, cells did not loose/gain mass well. My results didn’t vary a lot for the same concentration. I tried to make sure that the start mass was the as close as possible to the same as the others in the hope that the change in mass would be roughly the same. In some cases this did not happen but it was generally only to make the experiment easier and more understandable when reading the results.
My results did lie close to the curve, and therefore were fairly accurate, but if I was doing the experiment again I could make changes, some as mentioned above. I could weigh each chip on a digital and more accurate scale, e.g. not to 0.00 but to 0.0000g.
I could extend my enquiry by testing the percentage change in mass with concentration using a different substance. By this I mean using a different vegetable, perhaps celery or cucumber. Then I could find out whether osmosis occurs with the same patterns and trends with any vegetable.
I only really found one anomalous result and that was for the 0.6 molar solution
I could’ve also taken results with other substances dissolved into the water, such as sugar.
I could have taken results of more concentrations. This way I could have found the isotonic point by there being no change in mass. This would show the water potential of the potato, and this result could then be used further. Also to make my experiment better I could have repeated it more. I could have also used cloned potatoes, so that they would have all been the same. I could have also cut the potatoes into ring doughnut shapes, because the cells in the centre of the potato may have a different capacity of water and the intake, or distribution of it, or different water potentials. If I had cut the potato into doughnuts, I would have only been using the out side of the potato, leaving out the middle. I could have also used a machine to cut the potato chips.
