The chips that increase in mass have taken on more water which means that at the beginning of the experiment the sucrose solution had a higher concentration of water than the chip and this is likely to apply to the weaker sucrose solutions as they have a low concentration of sugar and therefore a high percentage of water molecules. The strong sucrose solutions have a higher percentage of sugar molecules and therefore a lower percentage of water molecules and if the concentration of water in the potato chip is higher than in the solution then water will leave the potato chip by osmosis and therefore the chip will lose mass. The water molecules move into cells that contain a lower concentration due to the difference in osmotic potential between cells. The water molecules move through a semi-permeable membrane (the cell membrane) which has big enough holes in it to let through the small water molecules, but not the bigger sucrose molecules. At a certain concentration of sucrose solution the concentration of water will be the same in both the chip and the solution which means there will be no transferral of water between the two and therefore no change in mass of the potato chip. This point is called the equilibrium.
Apparatus:
5 boiling tubes
15 equally sized potato chips
60ml each of 5 sucrose solutions of concentration 0M, 0.1M, 0.2M, 0.3M, 0.4M,
Boiling tube rack
Digital weighing scales accurate to 0.01g
6 paper towels
Ruler
Stop watch
Measuring cylinder
Method:
Cut 5 potato chips equal in size at 50mm long.
Using a dry paper towel wipe off any excess water on the potato chips.
Weigh each potato chip and record it’s mass.
Measure out 30ml of each solution and pour each one into a boiling tube. Rinse out the measuring cylinder between different concentrations so a residue of sugar does not build up and affect concentration of solutions.
Place the potato chips in separate boiling tubes simultaneously.
For each boiling tube make sure to keep record of the original mass of the potato chip and the concentration of the solution in it.
Time the experiment for 30 minutes and when the time is up pour out the solutions into a sink simultaneously.
Make sure that in each test-tube the temperature and the volume of water is kept the same and that the only variable that changes is the concentration of the solution. The temperature and the volume of the sucrose in each test tube must be the same because if the temperature of a solution is higher then the particles will move around more quickly, and therefore might increase the rate of osmosis because water molecules will be moving about and hitting the cell membrane more often. If the volume of solution is higher than it should be, then after a certain amount of osmosis the percentage and therefore concentration of water in the solution won’t have dropped as much as it would have if there had been a smaller volume of solution, and therefore the rate of osmosis will be faster.
Using a dry paper towel wipe off any excess water on the potato chips.
Weigh each potato chip and record it’s new mass.
Subtract the original mass from the new mass to determine the change in mass. A positive number denotes an increase in mass while a negative number denotes a loss of mass.
For each potato chip be sure to keep a record of the concentration of solution it was in.
Clean out all of the boiling tubes and the measuring cylinder and repeat this experiment twice more in order to have three sets of results and a more accurate set of averages.
Results:
These are my raw results.
In order to account for the differences in mass of the original potato chips I have decided to display my results for the final mass as a percentage change from the original mass.
Average
I can find the equilibrium on the graphs by looking at where the line of best fit intercepts the x axis. Each experiment showed the equilibrium to be at different places. Experiment 1 showed the equilibrium to be at 0.24M, experiment 2 showed it to be at exactly 0.2M, experiment 3 showed it to be at 0.16M. Therefore my average equilibrium for the three sets of results is exactly 0.2M. The gradient of my line of best fit for the average results is -0.3.
Conclusion:
It can be seen from my graph that as the molarity of a sucrose solution increases, the final mass decreases, hence the negative gradient of the line of best fit.
My results show that the rate of osmosis in plant tissue is directly affected by the concentration of the solution surrounding it, and as the gradient of the straight line of best fit is negative, the two have an inverse linear relationship. This means that regardless of the molarity of the solution, a given increase in the molarity of the solution (independent variable) always causes the same size of decrease in the dependent variable (percentage change of mass of potato chip).
As my line of best fit is linear and negative it shows that as the concentration of sucrose in a solution increases less water molecules pass into the plant cells or more water molecules move out of the plant cells. The point at which there is no movement of water molecules either in or out of the plant cells is called the equilibrium, and at this concentration of solution the concentration of water molecules is the same in the solution and in the plant cells, and therefore there is no need for any movement of water molecules.
My results support my prediction that two or three of the five potato chips would increase in mass while the others decreased and also that the equilibrium would be around 0.2M as my preliminary results suggested. As I had hoped in preparation for my experiment, I found the approximate equilibrium (0.2M) which is where there is no change in mass of the potato chip because the concentration of water in the solution and the potato chip are the same. My results support my prediction and the theory of osmosis because water molecules always passed from a high concentration of water molecules to a low concentration as I predicted, and that is what should happen according to the theory of osmosis. Therefore I found that the molarity of the potato chip is 0.2M. My results have certainly shown that osmosis works in plant tissue because there as a measurable change in mass of the potato chip. From my results I can conclude that water does move in or out of plant cells from a low concentrated solution to a higher concentrated solution.
Evaluation:
I think my experiment went very well because my results and my graph looked as I had expected them to, and I didn’t have any severely anomalous data. I think my results are pretty accurate, as they lie pretty close to the line of best fit, but they weren’t all exactly on the line of best fit. This could be due to the potato chips being from different potatoes or the chips being of different age. To make my experiment more reliable I could make the potato chips exactly the same shape and size to ensure that each one has exactly the same surface area, which would mean that the experiment would be very fair. However the way I cut the potato chips (to the same length) seemed to be accurate enough as my results showed, and taking much more care in cutting the potato chips would probably have little effect on the results. Because my potato chips were all slightly different in mass I calculated the percentage of mass lost or gained as opposed to the actual mass in grams as a way of getting around this inaccuracy.
It would appear that my experiment was fairly reliable because I did three experiments and all three sets of data showed similar results and similar gradients of line of best fit and there were no anomalous pieces of data. As my results look fairly reliable and accurate, it is fair to say that they support my conclusion about the link between the molarity of a solution and the loss or gain in mass of the potato chip over a set period of time.
In further experiments I would like to use solutions from 0.15M to 0.25 molars with a different solution at every 0.01M in order to get a more precise and accurate idea of the equilibrium. I would also like to do an experiment using the same concentrations as I did in this one, but repeating it about 5 times, each time leaving the potato chip in the solution for different time periods. I could then compare the gradients of the lines of best fit for the 5 different times, and also draw graphs for each molarity across the 5 time periods. I could also do an experiment using the same concentrations as I did in this experiment, but measuring the mass of the potato chips after every 3 or 4 hours until the mass stays the same, and see how long potato chips in different solutions took to reach a final mass and to see how large it’s mass would get. Finally I would like to do the same experiment as I did here, but try it out on different types of plants and compare the rates of osmosis of the different plants. This would give an idea of which plants were more efficient at taking up water and I could see what types of plants had the fastest rate of osmosis, and whether there was a link between the rate of osmosis in a plant and the habitat it exists in. For example I might find that plants that live in hot, dry conditions have a faster rate of osmosis than plants which live in cold, wet environments. These experiments would help give a better idea of how the rate of osmosis is affected by the concentration of a solution.
