These preliminary results show me the kind of results expected. If I have drastically different results then I will know that there has been a mistake made somewhere in the method.
Planned method:
A range of salt solutions will be prepared with concentrations 0.00 molar, 0.25 molar, 0.5 molar, 0.75 molar and 1.00 molar. Sections of potato will be cut using a scalpel and will be measured using a ruler. All of the potato slices will be as close to 15mm as I can make it. This must be done accurately as a change in the surface area may change the results. The mass of each potato will be taken so that more results can be obtained. Three slices will be placed on a wooden pin in each tube each time so that I can take an average for each tube. I will use 25cm³ of each concentration of salt solution and once in the test tubes they each will be labelled. The potato pieces will then be left overnight, for 25 hours. Then the potato pieces will be removed, the excess solution absorbed using paper towels and then they will be re-weighed.
Skill Area O: Obtaining evidence
Method:
1. I took a potato and washed it.
2. Using a cork-borer and scalpel I cut potato slices that were 15mm long and 10mm wide. I made 15 slices.
3. I measured and weighed each slice, weighing them in a particular order to ensure I knew which one was which.
4. I took 5 specimen tubes and labelled each one with my initials and the molarity of the solution.
5. Using a pipette and pipette I measured out different amounts of salt solution and distilled water which I then put into the test tubes in a percentage ratio giving me the various molar concentrations.
6. I put 3 potato slices into each beaker and then sealed the tubes. I sealed the tubes to prevent anything from getting into them and also to stop water from evaporating
7. Whilst waiting I drew up a basic table for my results.
8. After 25 hours I drained out the solutions in the sink and placed all the slices on the paper towel in the order I had put them in the test tubes as to not confuse myself as to which slice came from which solution.
9. I dried each slice with the paper towel and then placed each one on the scales so that I could weigh them. I recorded the weights and put them in the table with the previous results.
Results:
The attached graph gives the line of best fit for the percentage change in mass of the potato. The graph is a downward curving line. Because the line is not straight it means that the percentage gain and loss in mass and molarity are not directly proportional. Although there is a pattern on my graph: as the concentration of the solution increases, the percentage change in mass decreases. The graph shows that the percentages gain and loss in inversely proportional to the concentration. The gradient does change in my graph. It gets less steep as X axis gets bigger. This is because the potato is becoming flaccid and so the change in mass of each molar concentration is becoming closer and closer together. The graph shows the results accurately and from this you can see that the result for 0.75M does not fit, it is an anomaly. This could be because of various reasons such as contamination: I could have accidentally got something into the tube or I could have made a mistake when making up the solutions.
It shows that the potato cells increase in mass in solutions with a high water concentration and decrease in mass in solutions with a low water concentration. This graph of the change in mass helps prove the point of complete plasmolysis, whereby the potato cannot expand and take in any more water. Plasmolysis occurs when a plant cells membrane shrinks away from its cell wall. This happens when water is removed from the cell into the outside cell fluid. The movement of water occurs across the membrane moving from an area of high water concentration to an area of lower water concentration outside the cell. As you can see as the molar concentration increases the change in mass decreases. From right to left the first two points on the graph are very spread out indicating that there was a large change in the mass. This decreases throughout the increasing molar concentration until the change is very small.
This graph attached shows a clear indication that there was an overall decrease in mass during the experiment. At the point 0.00 M the line for after the experiment is above the line for before the experiment unlike any of the others. This is because the water potential of the salt solution is higher than that of the potato.
Skill Area E: Evaluation
The experiment was very successful in my opinion. I obtained accurate results from which I was able to create graphs. I think I took enough results for the amount of concentrations that I was using, and the time that I left the experiment for was enough to allow sufficient osmosis to occur. However if I was to repeat the experiment I might well increase the time of the result to allow more osmosis to happen and possibly find out the saturation point of the potatoes. The range of concentrations was adequate but I would possibly create more concentrations if I repeated the experiment so that I would have more varied results, i.e.0.1, 0.2, 0.3, and so on. This way would have allowed me to get more accurate results and would have allowed me to produce a more detailed graph.
The cutting of the potatoes was the most difficult part of the experiment as although I was recording my results by mass, it could well have affected the surface area and so the overall rate of osmosis. If I were to repeat the experiment I would have possibly found a machine to cut the potato as it would ensure that all potatoes would be the same weight and dimensions. As well as the potato, I could have found a more accurate way to measure out the solutions and to determine the molar concentrations. I could also weigh each slice on a more accurate scale, e.g. not to 0.00g but to 0.0000g.
There was only one anomaly and this may have been caused by me. When the potato slices were removed from the test tubes and dried I may well have dried some potatoes more thoroughly than others and so some would have more excess water, which would add to the mass. If the experiment was repeated I could find another way to dry the potatoes that would ensure that all were dried in the same way for the same time. However with all this said I think that the experiment was truly successful and I was very pleased with the complete comparison of my results with my initial prediction.
