Prediction
I predict that the potato tissue in the distilled water and weak salt solutions will gain weight and the potato tissue in the salt solution will lose weight because of osmosis. In osmosis, a high concentration of water moves to a low concentration of water. In the distilled water, there is a higher concentration of water than in the potato tissue so the water molecules move from the water to the potato tissue so the potato cylinder gains in mass and becomes turgid.
In the salt solution, the potato tissue has a higher concentration of water in the potato tissue than in the salt solution so the water molecules move from the potato tissue to the salt solution so the potato cylinder decreases in mass.
However, at a certain point, in both cases, the potato cylinders will not gain or lose any weight because the concentration of water will be the same in the cylinder and the solution. The cells in the potato will become flaccid and the cylinders will go floppy.
Variables Table
Before I plan my experiment, I need to consider the following variables:
Plan
Apparatus
My apparatus list will be the same as my preliminary apparatus with the following changes:
- 18 test tubes
- Cork borer size 3
- 6 test tube racks
- salt solution 1M/dm³
I will set up my experiment the same as my preliminary experiment.
Method
- I used the table above to make up my solutions making sure I took all my measurements off a flat surface and from the bottom of the meniscus.
- At each of the concentrations (see above table) I put 20cm³ of the solutions into 3 test tubes. I am using 3 test tubes for each concentration so I can take an average to get more accurate results and this will also allow me to repeat any anomalous results.
- I labelled each of the test tubes with the concentration and A, B or C.
- I cut 18 cylinders of potato from the same potato (see variables table) using a size 3 cork borer. I cut off the ends to get rid of the potato skin. I then cut each cylinder to 4cm using a knife.
- I dabbed off any excess liquid (see variables table).
- I put one cylinder into each of my test tubes and covered each test tube with cling film (see variables table).
- After 1 hour 30 minutes, I took out each of the cylinders, dabbed off excess solution and reweighed each cylinder using the same set of scales (see variables table). I then recorded the mass of each cylinder and put the cylinders back into the same test tube that they had come from, covering every test tube with cling film.
- After 4 hours since I originally put the cylinders into the test tubes, I took out the potato cylinders again, dabbed off excess liquid and reweighed each cylinder again.
- From my results I calculated the average of the original mass, average mass after 1 hour 30 minutes and the average mass after 4 hours. I also calculated the change in mass and the percentage change in mass. I used the following formula:
Percentage change in mass = Change in mass _ x100
Original mass
Analysis
I have looked at my results and drawn a graph from them. In my prediction, I stated that the potato cylinders in the distilled water solutions would gain the most in mass and the potato cylinders in the high percentage salt solutions would reduce in mass. From looking at my results, I found that the percentage change for 0.00M/dm³ was +22.5%. Also the percentage change for 1.00M/dm³ was -30.2%. This shows that this part of my prediction is correct – the weakest solution had the biggest increase change in mass and the highest concentration of salt solution had the biggest decrease change in mass.
These results were due to osmosis. This is because in osmosis, a high concentration of water will move to a lower concentration of liquid. In the distilled water solutions, there was a high concentration of water in the solution compared to the potato so water moved from the solution into the potato through the selectively permeable membrane therefore increasing the mass and the cylinder becomes turgid. In the salt solutions, there will be a higher concentration of water in the potato cylinder compared to the salt solution and so water will move from the potato cylinder into the salt solution therefore the mass of these potato cylinders will decrease. The potato cylinder will become flaccid. In my prediction I have included diagrams which can be used to further explain my results.
In my prediction, I also predicted that at a certain point there will be no gain or loss in mass because the concentration of water will be the same in the potato cylinder as in the solution. From looking at my results table, I can see that this point will be somewhere between 0.20M/dm³ and 0.40M/dm³. I can get a more accurate reading from looking at my graph. If I look at the line of best fit, the point where there is 0% change in mass is 0.24M/dm³.
Referring to my results table and graph, after 0.8M/dm³, a more concentrated solution loses less of it’s mass than 0.8M/dm³. On my graph, after 0.8M/dm³, the line of best fit slightly goes up. In my results table, the percentage change for 0.8M/dm³ is -31.6%. I would have expected the reading for 1.00M/dm³ to have a bigger percentage decrease in mass however the actual reading is -30.2%. This is because after a certain point, the potato cells are completely plasmolysed. Inside the cells there are now vacuoles. The salt solution fills these vacuoles by going through the fully permeable wall resulting in the mass starting to increase slightly.
- Dilute solution – Turgid
- Same Concentration of Solution
- Concentrated Solution – Flaccid
- Plasmolysed Cell
In my prediction, I did not expect this to happen, I expected that the 1.00M/dm³ would be the solution with the biggest percentage decrease in mass. However looking at plasmolysis explains why this happened. Therefore my prediction was right however I did not account for plasmolysis.
Evaluation
The first point of my evaluation is that I had an anomalous result – B at 0.00M/dm³. This is highlighted in red on my table. There are a number of reasons why this may have happened where I may have not controlled one of my variables. (see variables table). However from looking at the possibilities in my variables table. I used the same variety of potato. I left all my cylinders in the solution for the same amount of time, I used the same scales, as the concentration of this particular result is 0.00M/dm³ I think it is highly unlikely I used the wrong concentration of solution. I covered all my solutions with cling film carefully to stop evaporation. It could have been affected by diffusion however I placed all my test tubes in the same place so it would have affected all my results. I could have forgotten to dab my potato dry however, from looking at my table, I can see that the original mass was already too high before I had even put my cylinders in the solution. Therefore I think it likely that I either misread the reading on the scales or I did not cut this cylinder accurately enough with my cork borer and knife. I did not include this result in my averages, change in mass or percentage change as this would give me less accurate results. To improve my experiment I could have repeated the experiment at 0.00M/dm³ and use the new set of results.
In general, improvements, I could have made are:
- Do more repeats at each solution giving me a more accurate average.
- Repeat the experiment at 0.10M/dm³, 0.20M/dm³, 0.30M/dm³, 0.40/M/dm³ through to 1.00M/dm³ rather than just 0.20M/dm³, 0.40M/dm³, etc.
- I could have used a water bath to manage the temperature, controlling the rate of diffusion to make a fairer test.
- Used a different shape potato piece, e.g. a flat disk rather than a cylinder as this would give a larger surface area giving me more accurate results and a better average.
- In general, taken more time to be more accurate in preparation and in taking my results.
Although I could have made improvements, I had just the one anomalous result. I think that the main improvement I could have made was to make sure that my cylinders were all the same diameter and length to begin with which would have given me better averages. However from looking at my results I can say that they are reliable as I have taken 3 readings at each concentration of solution and my results table shows that all 3 readings are usually close together. It is very unlikely that all 3 results would be wrong and so I can be fairly sure that I have a set of reliable results.
