Preliminary Work and Research:
A preliminary experiment was set up in order to determine the range of values to be used in the main investigation (potato sample weight, concentration range).
The fact it was set up mainly for this reason, coupled with the time restrictions of only having two 50 minute sessions, meant that the results from the experiment were not needed to be very accurate. However, the main investigation will be completed to a larger degree of accuracy as the purpose of it is to find an exact and precise value (that of isotonic concentration).
Taking this need for only a fair amount of accuracy into account, the preliminary experiment was completed in the following stages:
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Use a SOMETHING to cut out potato samples from potatoes. The potato samples were taken from the same batch of potatoes so their contents would roughly be in the same proportion.
- All of the potato samples were then weighted using the same SCALES MACHINE, and their mass was then recorded.
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11 10ml solutions with varying concentrations were created. The concentrations of the solutions were the following- water, 9 ml concentrated solution with 1 ml water, 8 ml concentrated solution with 2 ml water, 7 ml concentrated solution with 3 ml water, 6 ml concentrated solution with 4 ml water, 5 ml concentrated solution with 5 ml water, 4 ml concentrated solution with 6 ml water, 3 ml concentrated solution with 7 ml water, 2 ml concentrated solution with 8 ml water, 1 ml concentrated solution with 9 ml water, Full concentration. The concentrated solution always remained the same and was obtained using a pipette.
- Place each sample in its own solution in their own test tube.
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Take out samples from their solution, reweigh them and record. This was done 2-2 and half hours after setting up the test tubes. The reason they were not reweighed at a set time after setting up the test tubes, because they could only be reweighed at a convenient time during a lessons break.
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Calculate the percentage gain/loss and plot on a graph.
Preliminary Results:
Preliminary Results Graph:
The preliminary experiment has also served to support and justify the prediction and theory of the experiment further. It has done this by displaying the negative correlation (between % gain/mass and sucrose concentration) that I expected there to be because of the reasons stated in my theory. The success of the preliminary work indicates that the basics of this experiment can be re-used for the main investigation with very few changes. The changes that will be made from the preliminary experiment to the main investigation will be there to enhance the accuracy of the main investigation.
The preliminary experiment results also suggest that the range of concentration values should remain the same, since they give a sufficient line of best and have good correlation. However, the line could be further improved by adding more results within this range.
Research:
Once the preliminary experiment had finished, I discovered the exact concentration of the sucrose solution to be 0.5g sugar/ 1ml distilled water.
Arbitrary figures were used in the preliminary experiment because the purpose of the preliminary was to judge the line of best fit and to find methods of doing the main investigation as efficiently as possible; but since the main investigation needs to find an exact figure (that of the isotonic concentration), precise and exact figures must be used.
TIME TAKING
Main method
To accurately measure only the affect of the varying input variable (which was the sucrose concentration of the solution), every other input variable had to remain as identical as possible. This was because they may affect the rate or amount of osmosis that takes place. For that reason, steps were taken to ensure that factors which are known to affect osmosis - test tube temperature, surface area of potato plant sample – were kept identical for each test tube.
Temperature affects osmosis because heat energy excites water molecules, and gets them to collide, disperse and diffuse more quickly and efficiently than they would without heat energy. Not keeping the temperature the same for each test tube would cause in unfair results because some potato samples will gain/lose mass quicker than others because of the varying rates of osmosis.
Temperature was kept the same through the following steps:
- All the test tubes were set up in the mornings of a single week so the temperature could not have varied that much.
- All the test tubes were set up in the same part of a room, so every one would receive the same amount of heat energy from the sun.
If the potato samples had a varying surface area, more osmosis would occur quicker in the ones with a larger surface area; and less osmosis would occur slower in the ones with a smaller surface area. Therefore, the surface areas also had to be kept the same. This was done by using the same SOMETHING to extract potato plant samples. This meant that all the potato plant samples had the same length, diameter and height.
Consequently, this action caused the mass of the potato plant samples to be near identical as well. This was shown as the masses of each potato sample were recorded using a scales machine before they were placed into the solution. Though mass is not known to affect osmosis, the fact it was kept the same did increase the accuracy of the results because there is a possibility that differing masses would have affected the rate of osmosis.
During the preliminary work, I found it difficult to place the potato samples in the test tube because the potato samples were very tall. This led me to cut all the potato samples by the same length so they could fit more easily into the test tubes.
Their surface areas remained identical because they were all cut by the same length.
The same brand, size, and type of test tube were used also to also prevent them from having any affect on the results.
The same volume of solution, which is 10ml, will be placed in each solution to prevent varying volumes from harming the accuracy of the results.
Arbitrary figures were used in the preliminary, so the inclusion of tap water- which may have its own sucrose concentration- to create varying concentrations did not matter. However, the main experiment needs to use exact figures (unlike the preliminary which only had to loosely prove a predicted trend). Therefore, a solution which definitely does not have a sucrose concentration will be used to dilute the sucrose concentrated solution. This solution will be distilled water.
How the various concentrations were created
Two 1000ml containers were set up; one was filled with 500g sugar/1000ml distilled water, and the other was filled with 1000ml distilled water.
From these containers, certain volumes (adding up to 10ml) were extracted and put into separate test tubes to create the following 11 differently concentrated solutions:
- 0.5g sugar/ 1ml distilled water full concentration (10ml from the concentrated solution container)
- 0.45g sugar/ 1ml distilled water (9ml from the concentrated solution container, 1ml from the distilled water solution)
- 0.4g sugar/ 1ml distilled water (8ml from the concentrated solution container, 2ml from the distilled water solution)
- 0.35g sugar/ 1ml distilled water (7ml from the concentrated solution container, 3ml from the distilled water solution)
- 0.3g sugar/ 1ml distilled water (6ml from the concentrated solution container, 4ml from the distilled water solution)
- 0.25g sugar/ 1ml distilled water (5ml from the concentrated solution container, 5ml from the distilled water solution)
- 0.2g sugar/ 1ml distilled water (4ml from the concentrated solution container, 6ml from the distilled water solution)
- 0.15g sugar/ 1ml distilled water (3ml from the concentrated solution container, 7ml from the distilled water solution)
- 0.1g sugar/ 1ml distilled water (2ml from the concentrated solution container, 8ml from the distilled water solution)
- 0.5g sugar/ 1ml distilled water (1ml from the concentrated solution container, 8ml from the distilled water solution)
- 0g sugar/ 1ml distilled water (10ml from the distilled water solution).
This range of concentrations, 0.5mls sucrose/1ml distilled – distilled, was used in the main experiment because it proved a success in giving a good line of best fit in the preliminary experiment.
The extractions from the containers were completed using a pipette. A pipette was used because it has a large degree of accuracy due to its small meniscus. The larger the meniscus, the more difficult it becomes to measure exact values – so the pipette was the ideal piece of equipment to use in this situation as I am measuring many exact values.
Collecting Evidence
Once each test tube was set up with their solution and potato sample, they were given exactly 2 hours for osmosis to take place. Afterwards, all the potato samples were taken from their test tubes (using a special PLUCK THING for precision) and re-weighed. During the taking of these results, a sketch scatter graph was drawn which displayed the anomalous results that needed to be re-done. The line of best fit from this scatter graph also indicated the area where the isotonic solution was (which was stage 3 in my predicted graph). To help me discover the exact isotonic solution, I decided to do more readings in this area. The entire second set of readings was completed the following day, in exactly the same process previously described for the first set. Another reason for doing extra readings is that the more readings, the more accurate the results. Therefore, as many results as possible were completed in the two 50 minute time session – so the results were as reliable as possible.
All the results from the experiment are shown in the table below:
Key for Table
NORMAL
ANOMALOUS RESULTS
RE-DONE BECAUSE OF ANOMALY
RE-DONE TO FIND EXACT ISOTONIC VALUE
Conclusion
The strong negative correlation of the results is due to the fact that as sucrose concentration increases beyond the concentration of the potato plant samples, more water molecules from the potato plant samples are needed to dilute it by osmosis.
This also means that the greater the contrast in the concentrations of the two solutions (created solution and the potato plant samples), the more the osmosis that takes place.
The main experiment has strongly proven that theory and prediction were correct fully, by giving the strong negative correlation that was predicted.
Above the x-axis, the potato plant cells are becoming turgid and gaining mass. They are doing this by osmosis- the water molecules travel from the solution to the potato, because the potato sample’s cells are hypertonic to the solution.
Where the readings are below the x-axis, the potato plant samples’ cells are becoming flaccid, and losing water molecules because the potato sample is hypotonic to its surrounding solution.
Where the line of best fit intercepts the x-axis, the potato plant sample cells are isotonic to the solution surrounding it. According to the graph, this solution is 0.15g sugar/1ml distilled water - 0.175g sugar/1ml distilled water.
Evaluation
Overall, the experiment has been very successful in proving the prediction. The results can be trusted, despite the few anomalies, mainly because it complies with the scientifically proven fact that the greater the contrast in concentrations, the more the osmosis that takes place. Also, the isotonic value, which was discovered to be between 0.15g sugar/1ml distilled water - 0.175g sugar/1ml distilled water is very close to the average of 30 other peoples isotonic value – which was BLAH. These 30 people completed the same experiment in generally the same way.
From observing the graph, it is clear to see that the circled points are clearly anomalous because they are very far away from the line of best fit. Including these readings as a part of the experiment would have negatively affected the correlation, so these results were re-done. The new results did successfully fit in with the trend of the graph.
The reason why anomalies occurred, and the reason for the two variables not being in direct inverse proportion is because it was impossible to fully exclude all external factors from affecting the experiment. For example, it was impossible to make each potato sample identical in every single way; so all of them would have at least slightly different sucrose concentrations and would have individual isotonic solution values.
To enhance the accuracy of the experiment, water baths, to keep each test tube at the same identical temperature, could have been used. Also, the re-weighing of the potato samples should have occurred at a set time after the experiment so some samples do not have extra time over others for osmosis to happen.
To further prove the experiment was successful, I could create a solution with the concentration 0.175g sugar/1 ml distilled water using the same procedure described earlier, and then place a potato sample in it to see if there is no gain/loss. If there is no gain/loss, the experiment would have been further proved successful.
If the investigation were to be re-done:
- a different vegetable would be used
- water baths to keep the test tubes at the same temperature would be used
- more readings with smaller intervals would be used to make the line of best fit more accurate, which would then make the x-axis intercept more accurate.