Preliminary Work
For my preliminary work I took a 50g slice of potato and I placed it in what I believed to be a heavily salted solution. I put 200cm3 of water in a bowl and placed 35g of salt in the bowl. I left the experiment for 24 hours and I returned to find that the potato had lost only a small percentage of weight. Not a significant amount that I could measure on my scales at home. I assumed the experiment had gone wrong and I was unsure why. The potato didn’t feel a lot softer or change colour.
Prediction
Osmosis is the passage of water molecules from a weaker solution into a stronger solution, through a semi-permeable membrane. In this case, the tiny holes in the membrane of the potatoes will allow the water molecules to pass through, and move both in and out of the solution and the potato, depending on the concentration gradient of the two substances. So in this case, when the water concentration is lower in the potato cell cytoplasm, the water will enter the cytoplasm of the potato, and the potato will gain weight as the cell size increases. If this was an animal cell, there would be no cell wall and the cell would burst. There is very little difference in the two water concentrations, there shouldn’t be such a big change in weight. And if there is a higher concentration of water in the potato, the water will exit the potato and the potato will lose weight, and shrink. If the cell had no cell wall then the cell would shrink a lot instead of the cytoplasm being stretched against the cell membrane. When the concentration is lower in the potato, the water will transfer from the solution to the potato. And vice versa.
In distilled water, I believe that the water is more concentrated in the potato, and therefore the water should transfer from the water to the potato, making the potato bigger in size, and heavier in weight. The potato cells, being surrounded by a weak solution, will be most likely to swell up and become turgid, taking in all the water it possibly can.
With the potato in molar solutions of salt, I think there will be a lot of change in the weight of the potato. I think that the maximum amount the cell can lose mass will be about 30% of its original weight. This is because there isn’t much difference between the two substances. I believe that the size of the potato won’t be affected much, but the weight will.
I think that the isotonic level of water to salt will be between the 0.4 molar solutions and the 0.2 molar solution. I think that in these solutions there will be a very small difference in the original weight of the potato. As the solution reaches 0.8 molars and 1.0 molar solution, the potato will discolour and will shrink. There may be a odd reaction or even many cells completely plasmolysing, due to the amount of water moving out of the cell. My theory in this experiment is that due to the difference in the water concentrations of the solutions, I believe that the weight of the potato will start decreasing after the 0.4 molar solution.
Safety
Safety is an important aspect in every experiment, even if the experiment seems to be very harmless. And that is why we take this into consideration.
We will be using a knife, which could injure someone if it’s not used correctly. We will also be careful that the solutions are not consumed as salt water can also be harmful to our bodies.
But other than that, there aren’t any other matters to be cautious of.
Method
I believe this task is in theory simple, but measuring out the solutions and getting the weights of the samples equal could be quite difficult. If the solutions are measured poorly then it could drastically affect our results.
There will be an amount of control in this experiment. The potatoes do not have to be exactly the same but must be similar. One set of experiments will be carried out on the same day and all the experiments will remain in the same room I the same place. Problems may occur as water could evaporate, also the room temperature could change on different days. There will not be a strict amount of control on the potato weight because of there is a 0.1g difference between them then it will not affect the weight change dramatically.
First, I will prepare the potato by peeling it. I will then push the apple borer through the potato, cutting into it lengthways. Then I will cut the cylinder into three equal lengths, which will all be altered until they, are all very similar. I have to do this twice so I have 1 cylinder for each solution. I will be doing 3 experiments so I need to do this 6 times. 1 cylinder of potato will be placed in one beaker of each molar solution. The solutions are left for 24 hours. The potatoes were placed in a 0.0 molar solution, a 0.2 molar solution, a 0.4 molar solution, a 0.6 molar solution, 0.8 molar solution and a 1.0 molar solution.
Each molar solution will be set up first and will then be ready for the potatoes to be placed into them. We decided to use only 503 of water or solution in each beaker, because the beaker capacity was just over 100cm3 and water could have been spilled or even overflow because of the added mass of the potato. This should not affect the experiment, it just makes it slightly more difficult though instead of measuring 20cm3 of water and 80cm3 of salt solution we would now have to measure half of this.
We took the potatoes out the next day and dried them so that osmosis didn’t continue and, so that the water left on the potato didn’t affect the weight or the weight of the other potatoes because some of the water may have come off and stayed on the scales, affecting all our results. We then weighed and recorded the result. We considered measuring how much water had left the beaker, but decided not to as some of the water could have been lost through evaporation. We repeated this experiment three times and recorded the average, and also the individual information about each experiment. We recorded the weight change, starting weight, final weight and also the weight change percentage. We worked out the average excluding any anomalous results. This ensures that our results are accurate. Repeating the experiment is important because if we didn’t then we would be unable to find any anomalous results, and also to make sure that the test was fair.
Table of results
In the table below, it shows the salt solution concentration column, which has the 6 different types of solutions. The experiment 1 column contains the results of each molar solution. This is the same in the experiment 2 and experiment 3. The final column is the average of the three tests. The increase/decrease of the weight is calculated by taking the weight after, minus the weight before. The percentage increase/decrease of the weight of the potato is calculated by taking the weight increase/decrease divided by the original weight multiplied by a hundred. This should help us lead to a more accurate result, and the average percentage will be used to present the graph. The average is worked out excluding all anomalous results. The table is shown below.
Analysis
By looking at my results I can see that my prediction was proved wrong. By looking at my results table I now predict that the isotonic level of water is between 0.2 and 0.0. By looking at the graph I believe that the solution is about 0.18. We decided then to expand the graph and test 0.16, 0.17, 0.18, 0.19 molar solutions. The results from this looked like this.
We couldn’t find the exact molar solution, as we are just a school and not highly paid scientists, and our equipment isn’t super accurate. We now believe that it is between 0.17 and 0.18.
There was a plateau in the graph. This occurred possibly, due to the semi permeable membrane of the potato cell having holes ripped into it, because water has moved out so fast. If there are holes in the membrane then osmosis cannot occur, because salt molecules can now move into the cell. Instead diffusion would happen and I believe that the graph result would be almost the same even if we tried a 2.0 molar solution. The plateau could also have occurred though because a cell can only lose so much water.
In the 0.2 molar solution we believed the 1st experiment was anomalous, and the other two experiments to be reasonable. But when we observed our results against the results of the whole class we realised that experiment 1 was correct and the experiments 2 and 3 were anomalous. We decided to borrow two results from the class, because if we had, had just one result, then this would have made the 0.2 molar solution unreliable.
I believe that my graph could be incorrect because of the amount of anomalous results. I think that some of the results are anomalous because either, we were inaccurate with our measuring, there may have been a problem with the weighing device, or maybe because we did the results on different days. The conditions could have changed and affected the amount of water in the beaker, some water could have evaporated.
Conclusion
In this experiment, I believe that my prediction, “I think that the isotonic level of water to salt will be between the 0.4 molar solutions and the 0.2 molar solution”, has been proven incorrect. Instead I believe that the isotonic level of water is between 0.17 and 0.18. The data that I have collected seems to prove this. Even though my conclusion was incorrect, I’m happy that this experiment worked successfully.
Evaluation
We followed the plan correctly, I believe we gained accurate and sufficient enough results to conclude the experiment, and to prove my prediction incorrect.
My final results were fairly reliable, due to the precautions we took to make this a fair test, but I’m a bit disappointed that the second experiment brought out 4 anomalous results. I believe that this could have occurred, because of poor measuring and also that the weather affected our results, because the temperature may have been significantly different, and affected water evaporation, making the water more concentrated.
To improve this experiment, I believe that we could have done all the experiments at the same time, so that the weather could not affect each experiment differently. We could also have used 1 litre of water, and a larger sample of potato. If the water samples were larger then it would help accuracy as an extra 2cm3 would not have affected the experiment very much. It would also have been easier to weigh the potato if the sample was larger, because the potato mass would have made it easier to measure the weight difference.
We could also have used the same potatoes because different potatoes may have different isotonic salt concentrations. This could also have caused the anomalous results in experiment 2.
The scales we used were also inaccurate and only measured to 1 decimal place, this made it harder to find exactly, how much weight had been lost or gained.
If we had more time to do the experiment we could have done the experiment many more times and found a much better and accurate average. Also we were unable to record our experiments exactly 24 hours later due to lesson times. If we worked in a laboratory, with carefully controlled conditions and better equipment, then I think we could have done this experiment a lot more accurately, and a lot quicker.
