Pilot Test:
I decided that before I carried out my investigation, it would be worthwhile doing a pilot test to get a rough idea of the results I might find, in order to help me create a hypothesis before I do my actual investigation.
For my pilot test, I used 33mm of potato. I carefully cut it to 33mm and weighed to around 1g, however I wasn’t too picky with accuracy, as it was only a pilot. I used 15mls of calcium carbonate solution- this was the amount needed to cover one piece of potato.
The first thing I decided to observe was how different concentrations affected the weight and length of the potato. I therefore needed to observe each end of the potato. I tried it in a 0.2 molar concentration, which would be my lowest of any concentration and 1.0 molar, which would be my highest.
I left them for 15minutes and saw that the results we collected were easy to compare because there was a range and an obvious reaction had taken place. I then used this pilot study to help with my scientific reasoning and with my hypothesis.
Predicting and Scientific Reasoning:
Osmosis is defined as the movement of water molecules from an area of high water concentration to an area of low water concentration, across a semi-permeable membrane.
In a high concentration of water the amount of solute is low. This is often known as a dilute solution. Whereas in a low concentration of water, the amount of solute is high and this would be called a concentrated solution.
When two such solutions are divided by a semi-permeable membrane the water will move from the area of high concentration to the area of low concentration, until both sides are equal. When they are equal they are said to have reached ‘equilibrium.’
This process can be seen in living cells. The cell membrane in cells is semi-permeable and the vacuole contains a sugar/salt solution. So when a cell is placed in a high water concentration, water will move across the semi-permeable membrane into the cell (lower water concentration) by osmosis, making the cell swell up and being ‘turgid.’
If this was done with potato cells the cells would theoretically speaking increase in length, volume and mass because of the extra water inside the cells. If these potato cells were placed in a solution with a low water concentration, then the opposite would happen. Water would move out of the cell into the solution. In some rare cases the cell membrane breaks away from the cell wall and the cell is referred to as being ‘plasmolysed’. The potato cells will have decreased in length, volume and mass due to the loss of water from the cells.
The main reaction is as follows: The greater the concentrations of water in the outer solution, the greater the amount of water that enters the cell by osmosis. The smaller the concentration of water in the outer solution, the greater the amount of water that leaves the cell.
However, there will be a point where the concentrations of water inside and outside the potato cells are equal, or ‘isotonic’. At this point there will be no change in the length, volume and mass of the potato, as the net movement of water will be zero, and no osmosis will have occurred.
I predict, that as I increase the concentration, the weight and length of the potato chip will decrease. My reasoning behind this, is that the higher the concentration of calcium carbonate in a solution, the lower the concentration of water. When the potato chip is put into the solution, it will lose some of its water by osmosis and the water will diffuse into the solution of glucose, causing the potato chip to lose water, thus decreasing in weight and length.
However, when I place a potato chip into a solution of 0 molar concentration, it should gain weight, width and length. This is because the solution has more water potential than the potato chip, and so water moves from a region of high concentration (the solution) to a region of low concentration (the potato chip.)
Whilst conducting my experiment I expect to come across a few minor detail problems, such as accuracy of the size of potato and the measurements of solution. I will therefore use a size 5 borer for each piece, which isn’t to thin it breaks the potato, but isn’t too thick that it won’t fit in the petri dish; size 5 is a decent size. I will then use the same ruler to cut each piece to 3cm, as accurately as I possibly can. I will also use the same potato for each piece in order to keep it a fair test.
After having conducted my pilot test, and thought about scientific reasoning I am now able to predict how a graph of my results may look. This will enable me to have something to compare my results to when I have finished the actual investigation. I can think of three different patterns, which the graph may follow, I hope that my final graph will have resemblance with one of them, as this will help me to see that my scientific reasoning has correlation with my actual results. On the graph I will also predict the isotonic point of the solution. In order for the graph to work, I will have to work out the percentage change of mass in each of the solutions with three replicates in each concentration. This will provide useful, and hopefully reliable results for the graph to be drawn up with. I predict that my graph will look like one of the following:
Out of these three graphs I think that the most likely is the S-shaped curve. This is because I know that osmosis occurs at different rates according to the membrane it is dealing with. I expect the first few concentrations to have a small change in mass, but then I expect a steep decrease in mass change percentage at around 0.2/0.4 M. I then expect the reaction rate, and overall change to slow down and decrease at a less steep gradient on the graph, as it approaches 0.6/0.8 M. This is because the osmosis will occur on different levels, depending on the concentration in the water. The water potential of pure water is 0, and as I add solute it will decrease. Water diffuses (osmosis) from where there is a high water potential, to where there is a low water potential. The potato will have a lower water potential than pure water, so at first the water, will diffuse into the potato. However, as I add calcium carbonate the water potential outside the potato will decrease so water will start to diffuse from the potato into the surrounding solution.
Initial Sizes and Weights of the Potato Tissue and the Solutions.
The initial pieces of potato I cut were cut out of a normal potato using a size 5 borer. I then cut the tissue up into tubes 3cm long. I took great care whilst doing this, to ensure that the pieces were all the same length and that the experiment would be fair. I used the same borer and ruler for each piece too.
In each petri dish I decided to put in 3 pieces of potato tissue. I then weighed each piece individually.
I used different amounts of water and sodium chloride in each solution in order to get accurate solutions and measurements. To make sure the solutions were accurate I used a measuring cylinder to measure the water and sodium chloride.
The pieces of potato tissue were all put into the petri dishes as 3:15pm on the 25th May. I was careful to have them all put in at the same time in order for it to be a fair test. The experiment was then left overnight.
Method.
Equipment:
- 6 petri dishes and lids
- Measuring Cylinder – 50ml
- Potato
- Size 5 borer
- Knife
- Ruler
- Top Pan Balance
- Sodium chloride
- Water
This equipment should be useful enough to gain accurate measurements and results from the experiment. I will use a 50ml-measuring cylinder because it is a round number, which I can easily work with to get the right concentrations. A size 5 borer will be appropriate because it enables the potato to be of an adequate diameter for changes to be seen. It isn’t too thin that the pieces will snap, and it isn’t too thick that they won’t fit in the petri dishes with the lid on. I think size 5 is the best size available. It is quite thin and therefore enables each of the cells to be reached so osmosis can occur quicker and to a more reliable standard. In a thicker piece it the solution may not reach the inner cells so a reaction will be less obvious. When weighing my pieces I will use the top pan balance and from it take a reading to two decimal places, this gives a reasonably good degree of accuracy, which won’t interrupt my results.
How I will do it:
- To begin with I will bore out a potato using the size 5 borer.
- I will then use the knife and ruler to accurately cut the tubes of potato tissue to 3cm long.
- Once the potatoes have been done and I have a total of 18equal pieces of potato tissue I will make up the solutions using the measuring cylinder and using the dilution levels on the previous page.
- I will pour each 50ml of solution into the petri dishes and put their lids on.
- I will then weigh the 3 pieces of potato going into each petri dish separately. For example, the three pieces in 0.2M will be labelled A, B and C, I will weigh each individually and take the results.
- Once each group of potatoes has been weighed I will place the three pieces of potato into each dish. In order to make it fair and have all the pieces going in at the same time I will ask my friends to help me place them in all at once.
- I will then leave the experiment for a period of time before coming back and taking out all the pieces at once.
- I will then weigh the 3 pieces from each dish collectively once more and measuring the diameter of each piece, writing down the results of each.
Diagram of Experiment.
Results.
I went back to my experiment at 9:00 am on 26th May. The potato tissue had been in the solutions for 17 hours and 45 minutes, long enough for a decent reaction to have taken place. I once again took out all the pieces at the same time, and then weighed each of the pieces.
The results were as follows:
I then used the formula %change = difference/start figure to calculate the percentage change of mass in each replicate.
I then decided that by finding the percentage uncertainty it would help with the analysis of my experiment and provide me with information as to how accurate my results were. To do this I used the formula: (½ range) / mean x 100
The results were as follows:
Analysis.
In my prediction I claimed that ‘as I increase the concentration, the weight and length of the potato chip will decrease.’ Having conducted my investigation I am now able to see that my prediction was correct. I knew that this would be the case because I know that water molecules diffuse from a high concentration (weaker solution), through a partially permeable membrane, into a lower water concentration (a stronger solution), as shown in these two diagrams:
This diagram shows how the molecules in a weak solution pass through the partially permeable membrane into the strong solution.
This diagram shows how the previously balanced amount of molecules in each solution change as osmosis occurs and molecules move from a high concentration to a low concentration.
I drew a graph from my results comparing the percentage change in mass against the concentration of the solution; this enables me to see what effect the concentration has on the average change in mass in each solution. My graph does, as I predicted, form the shape of an S. By looking at the isotonic line on my graph I can make a prediction as to what the concentration inside the potato is. I can get this by looking at where my line of best fit crosses the isotonic line; this is at approximately 0.29M. The graph shows that between 0.2M and 0.4M there is a higher percentage drop, which would fit accurately if the concentration in the potato is around 0.3M.
I found in my investigation that as I increased the concentration, the potato size decreased in correlation with its mass decrease. However there were restrictions on the decrease in size of the potato. The cell wall of a potato is rigid, as osmosis occurs it becomes more flaccid, but at a certain point this must stop, the cell wall has a point at which it will no longer decrease in size. I think that in relation to my graph, this point is at around 0.5M. I think this because it is at this point on the graph where the decrease in mass of the potato slows down. It still decreases from here to 1M, however, it is at a slower rate and I think this may be because the cell wall will not shrink any further, so the cells inside the wall are not necessarily affected straight away, as the cell wall may be quite a lot bigger than the fixture of cells inside at that point.
Looking at my results in the table I can see that there is a general trend following throughout most of the results. In the 0M solution the potato increases in mass by quite a bit, the increase in mass was then slightly reduced in 0.2M, then when it got to 0.4M the potato began to reduce in mass, until at 1M the weight was almost half the starting weight. It is clear that as the concentration is increased, the mass of the potato decreases. This must mean that as the concentration increases, the water molecules inside the potato diffuse out of the cell wall and into the solution. However when there is a low concentration, the potato increases in mass, this is because the concentration inside the potato is higher than that of the solution – as I already know, as it is likely that the concentration in the potato is, as the graph shows, 0.29M – so the water molecules in the solution will move through the partially permeable membrane on the potato, into the cells of the potato, thus increasing their mass.
Evaluation.
I think that on the whole, my experiment went fairly well. I managed to collect a variety of results for different concentrations and the effects they had on potato chips, which was the main aim of my experiment. In order to gain more accurate results and an adequate array I decided to use three pieces of potato in each solution. I think this was a better idea than just using one as it gave me more results and made them more reliable as I could compare them to others in that solution, being aware of any anomalies.
I did not come across to many problems in the planning stage of my investigation, however when it came to the practical part I started to encounter some difficulties in conducting the experiment. Although I managed to overcome these difficulties, they may have had an effect on my results if I had not sorted them.
My main difficulty was in cutting each chip to the same size. It was easy enough to cut the potato cylinders to the right diameter, as I used the same borer for each piece, however when it came to cutting the potato length ways I encountered some problems. I used a ruler to cut the pieces to 3cm, however because a ruler is only accurate to around 0.5mm, some of the lengths may not have been accurate enough. There are a couple of ways I could improve on the accuracy of my pieces, firstly I could have used a top pan balance, to gain more accurate measurements, or, a better and more reliable way, would have been to use a laser to measure each piece so that they were all equal.
In order to find out how reliable my results were I decided to do a percentage uncertainty on my results. I found that my percentages were quite interesting. For some reason I had a higher percentage uncertainty in the weaker concentrations, and a very low percentage in the high concentrations. The only explanation I have for this is that the levels, which the pieces decreased by in the higher concentrations, were closer together than that of the weaker concentrations, therefore the mean change was lower and the entire equation for the percentage uncertainty was lower.
If I were to repeat this investigation I think that I would spend more time, using more reliable devices, such as a laser, to attain more accurate measurements and therefore results with lower percentage uncertainties. This would improve the overall outcome of my investigation.
I could also do the experiment with different concentrations, and more of them i.e. 0.1M, 0.15M, 0.2M etc. This would provide a wider variety of results, which would make my estimate of the isotonic point, and concentration inside the potato, more accurate.
Overall I am pleased with how my investigation went. I collected the results I needed, could draw conclusions from them and could relate them to my predictions and the scientific reasoning behind them. There are improvements I could have made, which I am fully aware of, however as far as this experiment went I am satisfied with the outcome.
