Plan of Procedure
First of all I will bore out a cylinder of potato and cut it to 15mm. The diameter of the potato will be 6mm. Next I will measure each potato to make sure they are the same length with each other. I must make sure that each piece of potato is precisely the same length and, more importantly, weight so that my results are not inaccurate. If the potatoes are different weights then there could be more water in them. This would mean that in high concentration sugar solution the concentration gradient would be outside the potato therefore water would move from the potato to the sugar solution making the potato weigh less. I will measure out five potatoes because that is how many different solutions I have: 20ml 2 molar sugar solution, 15ml sugar solution with 5ml water, 10ml sugar solution with 10ml water, 5ml sugar solution with 15ml water and, finally, 20ml water. This gives me a good, even range of different solutions to make sure that my variable has been changed sufficiently. I must make sure that the solutions are accurately measured so that I do not get any two solutions the same. This would mean I would most likely end up with two results the same that would basically be a gap in my results. Then I will put a potato in each of the solutions while someone else starts the timer for me. The person must start the timer simultaneously when I put the potatoes in so as every potato is in for the same amount of time to the second. If they are in for any longer then they will have had longer for osmosis to take place so the weights would not be correct compared to the others. I will take the potatoes out of the cylinder and measure them after 5mins. The osmosis process stops when they are taken out of the solution so the time out of the test tube will not effect the results. I then replace them back in the test tube and take them out four more times at 5min intervals. This will mean the potatoes will have been in for a total of 25mins. At each interval I will weigh each potato and record each result.
Safety is a very important aspect of every experiment, even experiments that seem to be harmless. I will be using a very sharp that could harm someone if not used properly. I will ensure that when I have finished using the knife I will put it safely back so no damage can be done. You must always where goggles and an apron when doing a practical experiment so no substances could possibly get into your eyes or onto your clothes as I am not certain what they could do. I must also make sure that no one swallows the substances.
Prediction
For osmosis to take place there must be an imbalance in the concentration gradient of the two substances in question. In this case when the water concentration is lower in the tissue, the water will go inside the tissue of the potato, and the potato will gain weight. If there is very little different in the two water concentrations, there should not be such a big change in weight. If there is a higher concentration of water in the potato, the water will go out of the potato. So, when the potato is in the high concentration of sugar solution then the concentration gradient will go from the potato into the solution. This is because the semi-permeable membrane only allows through particles, which are small enough, for example water. The membrane does not allow through large particles, for example sugar.
Results
First Test
Second Test
Analysis & Conclusion
From my graph I can see that as the amount of water increased the amount of osmosis taking place increased. The 2 molar sugar solution produced quite a high decrease in the weight of the potato, as did the 1.5 molar sugar solution. The 1.5 molar solution decreased more than I expected. The increase in then weight of the potato was very slight for the 1 molar although I expected there to be no change. The all water solution and 0.5 molar sugar solution worked as I expected and the weight increased quite dramatically.
The trend shown here was that as the concentration of the solution decreases the increase of the weight of the potato increase. I believe that the trend shown here happened because osmosis only takes place when there is a concentration gradient. Because there was about the same amount of water in the potato as there was in the solution, there was very little change in the weight of the potato on the 1 molar solution; only 0.01 of a gram. In the test with 2 molar sugar solution the potato lost a lot of weight because there was more water in the potato than there was in the solution. This meant that the concentration gradient went from the potato into the water causing osmosis to take place out of the potato into the sugar solution. In the 1.5 sugar solution the decrease was a lot more than I expected it to be. I did expect it to decrease but not as much as it did. I expected it to decrease less than the 2 molar test but more than the 1 molar test. I think that I must have measured the amount of solution wrong for this result to come out. For the last two results I got just the results I was expecting. The potato increased in weight because there was more water outside the potato than inside so the water diffused from the solution into the potato as this was the direction of the concentration gradient. All of the results support my prediction apart from the 1.5 molar sugar solution. They supported my prediction because when there was more water in the solution; to balance the two out water diffused from the solution into the potato. Likewise when there was more water in the potato then the water diffused from the potato into the solution to try to balance them out. The result that supported my prediction but did follow the trend was the one for the 1.5 molar solution. It did what it should have done in that the weight of the weight of the potato decreased but it decreased to much; so that the results did not steadily increase as I expected. Overall my results support my prediction but the 1.5 molar result decreased too much, not following the trend.
Evaluation
I think that with the equipment given I was not able to make completely accurate measurements. When I was using the weighing scales, which are very sensitive even to the air, when people walked past quickly the weight would change slightly and so we would often have to reset the scales again. I should have been able to make very accurate measurements of the solution because as long as you made sure your eye was level with the, liquid it was fairly easy to see when the liquid was at the right place. I also think it was very difficult to measure the potatoes to the exact same length because if you got it a little bit wrong and you chopped off a little bit more than you should have then you would have to keep chopping off bits of the others and so you could never get the length of the results to be exactly the same. Although the result for the 1.5 molar did decrease it decreased to much and so was not inline with the others so I think this was anomalous. In think there are a number of reasons why this could have happened. One is that I did not measure the solution properly. We probably made the solution too concentrated which would have made there be even more water in the potato so there would have been an even bigger decrease than for the 2molar. Alternatively we could have made the potato too big so therefore there would have been even more water in the potato that needed to diffuse out of the potato to make it even. The last reason I could think of was that I had got the solutions mixed up and that the 1.5 one should have been the 2 molar solution. If this had been the case then the results would all follow the same pattern.
I think it was good that we used a cylinder for cutting out the potatoes as it made sure that at least every potato was exactly the same width, which was better than cutting each potato into squares. That would have made it even more difficult to get the same size potatoes. I think it was bad how we measured the weight of the potato as we could not always get the exact same weight for each potato. This could be why we got that anomalous result. If I did this experiment again I would take the weighing scales away from everyone else and weigh each potato much more accurately without any disturbances. Also when I had to put the potatoes into the solution and press the timer at the same time, because I did not have five hands this could not be done all at once although I was as quick as I could, they were not put in at exactly the same time. Next time I would get two more friends to help me so we could put each potato in at exactly the same time as when we pressed the timer. Also when we stopped the timer to weigh the potato after each 5 mins interval they could not be taken out at exactly the same time so some had a little more chance for osmosis to take place, which would have made it a little bit unfair. I think it would have been useful to have used a sharper knife as sometimes when we cutting the potato we had to saw a little making the end jagged instead of straight. Next I think I will also try using a cleaner measuring cylinder as the one I used was quite dirty so you could not quite see if your liquid was in the exact right place. Apart from these problems I think that my procedure was suitable and worked quite well and fluently.
I think that my results were quite reliable as they did follow the expected trend despite one of them going over the expected trend! There was only one anomalous result and even this was not very far out. The repeated results gave almost exactly the same results that I was very proud of. There was definitely a wide enough range of results to be able to analyse them well and thoroughly.
I think that to improve the reliability of my results I would have to measure everything much more accurately. A good variable to investigate related to this would be the amount of solution as this would help me to see what happened with my anomalous result.
- Word count: 2,544
- Sources: www.essaybank.com
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