- I decided to use larger pieces of potato in my real experiment, so any changes would be more obvious and so my measurements could be more accurate.
- I also chose to measure the mass of potato pieces using a set of scales with 2 decimal places of accuracy. This made my measurements much more accurate.
- I chose to repeat my experiment three times for each concentration, to make my results more reliable.
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Because of this, I placed all three pieces of potato in the same beaker (this time with 100cm3 of solution). This could mean that if the concentration of one of the beakers was slightly wrong due to human error, this mistake would impact on all three results. I decided it would be too impractical to use 18 separate test tubes.
- I kept the same intervals of concentration (0%, 20%, 40%, 60%, 80%, and 100%), as this gave my results a good range.
- I also used the same size of potato core and the same concentration of sucrose solution, although I don’t think either of these will affect my results in any way.
This is the method I used for my experiment:
- Set up the following equipment: 5 beakers each containing a different amount of sucrose solution and water:
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100cm3 of sucrose solution.
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80cm3 of sucrose solution, 20cm3 of water.
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60cm3 of sucrose solution, 40cm3 of water.
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40cm3 of sucrose solution, 60cm3 of water.
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20cm3 of sucrose solution, 80cm3 of water.
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100cm3 of water.
- Measure out 18 potato cores of 5mm diameter, which all weigh 1g within 0.02g of accuracy (ie they all weigh between 0.98-1.02g). This is because the scales fluctuated by around this amount just because of air movements, so I decided it would be impractical to ensure every core weighed exactly 1.00g.
- Place three cores of potato in each beaker. Then leave them for approximately 50 hours (within 15 minutes).
- To measure the potato cores, wipe off the excess water and then weight them. Work out the change in mass by taking away 1.00g from this value.
I will keep my experiment a fair test in the following ways:
- Keep the temperature of all the beakers the same (by keeping them close together in the room).
- The potato cores are all the same shape as well as the same weight, so they have the same surface area, in case it has any affect.
- Remove all the pieces of potato after the same amount of time, so none of them undergo osmosis for longer than any of the others.
- Use the same syringe to measure out the water and sucrose solution so that if the syringe has any imperfections in its accuracy they will at least be consistent for the whole experiment.
- Use the same scales to measure every piece of potato, before and after the experiment. Similarly, if the scales have any imperfections, these will be consistent throughout the whole experiment.
- Use sucrose solution from the same bottle in case it has been contaminated and the concentration is not exactly correct, so it will not affect the results.
- Use cores from the same potato so there are no differences in the sucrose concentrations of each potato core before the experiment.
Results
The first time I measured the 0% sucrose cores, there was a problem with the scales and it recorded 1.20g, 1.10g and 1.37g. However, after the scales were fixed and I re-measured the cores, I got these correct results.
Conclusion
From my graph, it seems that the change in mass in higher, the lower the percentage of sucrose, and the mass decreases with higher percentages of sucrose. The mean result for 0% sucrose was that the potato increased in mass by 0.37g. The mean result for 100% sucrose showed that the potato decreased in mass by 0.23g. The results in between these also support this trend.
My line of best fit is very close to 5 of my results. However, the result for 20% sucrose is off by quite a large amount. I think this result could be an outlier caused by an error in measuring the concentration inaccurately.
I think this pattern is caused because the concentration of the solution inside the potato cells is equivalent to around 50-60% sucrose solution. I think this because all of the values higher than 60% sucrose caused the potato to decrease in mass. This would be because the water inside the potato cells moved out into the more concentrated solution around it. All of the values lower than 40% sucrose caused the potato to increase in mass, because the water around the potato cells moved inside to the more dilute solution inside the cells.
Evaluation
These are some of the things that could have affected the reliability of my results:
- The biggest issue is the apparent outlier at 20% sucrose. I believe this was caused because the concentration of the beaker was slightly out due to human error. Because all three potato cores were in the same beaker, this didn’t just affect one result which could have been discounted; it affected all three, which means it also affected the mean. To improve this I would have separate potato cores in separate test tubes. This would improve the results because outliers would be more evident.
- When weighing the potato after the experiment, I didn’t dry off all the water; I just wiped off the excess drops. This could mean that the final masses are slightly higher than the true masses. However, this applies to all the weights, so the pattern is still reliable, even if the individual results are not perfectly accurate. To improve this I would dry the potato pieces on tissue or cloth before I weighed them, which would make the results more accurate.
Overall, I think my method and equipment was fairly accurate. My results were quite accurate and there were not many big improvements I would need to make to improve my method and equipment.
The lower three percentages have relatively large error bars, compared to the upper three percentages. I cannot think of an explanation for this, except perhaps there was less human error when weighing the last three because me and my group got more experienced with using our temperamental scales accurately. The only error bars which overlap are those for 20% and 40%, which I believe is due to the outlier of 20%. This means most of my results show a real difference, and there are enough accurate results not including those for 2-%, that I believe my experiment still shows a real pattern.
Overall, I am very confident in my conclusion. My method was good, and there are very few major improvements, and a small number of minor ones that I would make if I did the experiment again. My equipment and setup was the weakest area of my experiment, sue to the poor setup of placing the repeats in the same solution, and the slightly temperamental scales I used.
However, my data is fairly reliable, and mostly has small error bars which do not overlap and are close to the line of best fit. So overall, I think my conclusion is true and reliable.